Mirror device, mirror array, optical switch, mirror device manufacturing method, and mirror substrate manufacturing method

ABSTRACT

A mirror device includes a mirror ( 153 ) which is supported to be pivotable with respect to a mirror substrate ( 151 ), a driving electrode ( 103 - 1 - 103 - 4 ) which is formed on an electrode substrate ( 101 ) facing the mirror substrate, and an antistatic structure ( 106 ) which is arranged in a space between the mirror and the electrode substrate. This structure can fix the potential of the lower surface of the mirror and suppress drift of the mirror by applying a second potential to the antistatic structure.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a Divisional application claiming thebenefit of application Ser. No. 11/597,234, filed Nov. 20, 2005 now U.S.Pat. No. 8,149,489, which is a non-provisional application claiming thebenefit of International Application No. PCT/JP/2005/024078 filed Dec.28, 2005.

TECHNICAL FIELD

The present invention relates to an electrostatically driven mirrordevice having a mirror with changeable tilt angles, a mirror arrayhaving a plurality of mirror devices arranged two-dimensionally, anoptical switch having the or array, a method of manufacturing the mirrordevice, and a method of manufacturing a mirror substrate included in themirror device.

BACKGROUND ART

MEMS (Micro Electro Mechanical Systems) optical switches have received agreat deal of attention as a hardware technology to implementlarge-scale optical switches. The most characteristic component of aMEMS optical switch is a MEMS mirror array. The MEMS mirror arrayincludes a plurality of MEMS mirror devices (to be referred to as mirrordevices hereinafter) arrayed two-dimensionally. A conventional mirrordevice (see, e.g., Japanese Patent No. 3579015) will be described below.

As shown in FIGS. 107 and 108, an insulating layer 8002 made of asilicon oxide film is formed on a lower substrate 8001 of single-crystalsilicon. Four driving electrodes 8003-1 to 8003-4 are provided on theinsulating layer 8002 at the center of the substrate 8001. Supports 8004of single-crystal silicon are provided on both sides of the uppersurface of the tower substrate 8001.

An upper substrate 8101 has an annular gimbal 8102 inside. A mirror 8103is provided inside the gimbal 8102. For example, a Ti/Pt/Au layer (notshown) with a three-layered structure is formed on the upper surface ofthe mirror 8103. Torsion springs 8104 connect the upper substrate 8101to the gimbal 8102 at two 180° opposite points. Similarly, torsionsprings 8105 connect the gimbal 8102 to the mirror 8103 at two 180°opposite points. The X-axis passing through the pair of torsion springs8104 and the Y-axis passing through the pair of torsion springs 8105intersect at a right angle. As a result, the mirror 8103 can pivotaround the X- and Y-axes each serving as a pivot axis. The uppersubstrate 8101, gimbal 8102, mirror 8103, and torsion springs 8104 and8105 are integrally made of single-crystal silicon.

The structure of the lower substrate 8001 and the structure of the uppersubstrate 8101 shown in FIGS. 107 and 108 are separately manufactured.The upper substrate 8101 is soldered to the supports 8004 so that theupper substrate 8101 bonds to the lower substrate 8001. In this mirrordevice, the mirror 8103 is grounded. A positive voltage is applied tothe driving electrodes 8003-1 to 8003-4 to generate an asymmetricalpotential difference between the driving electrodes 8003-1 to 8003-4. Anelectrostatic force attracts the mirror 8103 and causes it to pivot inan arbitrary direction.

The design of the mirror device whose driving electrodes 8003-1 to8003-4 generate an electrostatic force to drive the mirror 8103 is basedon the fact that the electrostatic force is proportional to the secondpower of size, i.e., the area, unlike the gravity or inertial force thatis proportional to the third power of size, i.e., the volume. For theusual order of centimeters, the presence of an electrostatic force isnoticeable only in frictional electricity with a high voltage of severalthousand V or more. As the size reduces, an inertial force abruptlybecomes small in proportional to the third power of size. However, theelectrostatic force decreases in proportional to the second power ofsize. Hence, the electrostatic force can lift or move an object even ata low voltage of several V to several ten V in a microscopic world. Inthe mirror device shown in FIGS. 107 and 108, the diameter of the mirror8103 is, e.g., about 500 μm. The distance between the mirror 8103 andthe driving electrodes 8003-1 to 8003-4 is, e.g., about 90 μm.

Frictional electricity causes spark discharge due to its high voltage.In a small mirror device, however, no avalanche discharge with sparkoccurs with the same electric field strength. This is because even whenthe electric field is strong, particles (particles ionized due to somereason, e.g., ions in air that are ionized by cosmic rays or naturalradiation) accelerated by it cannot acquire energy so high as to ionizeother neutral particles collided with them because of the short distancebetween the mirror 8103 and the driving electrodes 8003-1 to 8003-4. Theelectrostatic force is proportional to the electric field strengthbetween the electrodes (in the mirror device, between the mirror 8103and the driving electrodes 8003-1 to 8003-4). Hence, if theinterelectrode distance is long, it is necessary to give a large voltagedifference between the electrodes. However, the large voltage differenceapplied between the electrodes may cause discharge, as described above.Even with the same electric field strength, the voltage differenceapplied between the electrodes can decrease in proportional to theinterelectrode distance in the small mirror device. Since theabove-described factor prevents discharge, a stable driving force isavailable. The reasons that mainly make the electrostatic forceeffective as a driving force in the mirror device have been describedabove. Use of the electrostatic force allows to control the drivingforce by the voltage applied to the driving electrodes 8003-1 to 8003-4.Since control by an electronic circuit is easy, and any steadily flowingcurrent does not exist, power consumption greatly decreases.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

The absolute value of the force necessary for driving the mirror 8103 issmall because the size of the mirror device is small. When using aninertial force as the driving force, the absolute value of the forcenecessary for driving the mirror 8103 abruptly decreases in proportionto the third power of size. An electrostatic force also decreases inproportion to the second power of size, although it is relatively largeas compared to the absolute value of the force necessary for driving themirror 8103. The mirror device that operates with a very small forcereadily receives the influence of an unexpected slight electrostaticforce that is negligible in a usual size.

A typical example is a drift phenomenon that poses a problem in anelectrostatically driven MEMS mirror device. Drift indicates adisplacement of a tilt angle θ of the mirror 8103 from a proper angledecided by the voltage applied to the driving electrodes 8003-1 to8003-4. Those skilled in the art have known the problem of drift for along time and understood that the cause of it is related to movement ofcharges. That is drift occurs in relation to a time required to chargebetween electrodes or another stray capacitance or electrification nearthe electrodes. However, a specific mechanism of drift is unknown, andtherefore, only measures are empirically taken against drift. That is,the measures can or cannot have effects, and it is hard to say that apractical method is established.

Means of Solution to the Problems

The present invention has been made in consideration of theabove-described conventional problems, and has as its object to suppressdrift of a mirror.

It is another object of the present invention to provide a mirror devicemanufacturing method capable of manufacturing a mirror with a desiredwarp amount.

It is still another object of the present invention to form a mirrorsubstrate at a high yield of non-defective units.

It is still another object of the present invention to achievelow-voltage driving and cost reduction and increase the pivot angle of amirror.

In order to achieve the above objects, according to the presentinvention, there is provided a mirror device characterized by comprisinga mirror which is supported to be pivotable with respect to a mirrorsubstrate, a driving electrode which is formed on an electrode substratefacing the mirror substrate, and an antistatic structure which isarranged in a space between the mirror and the electrode substrate.

According to the present invention, there is also provided a mirrorarray characterized by two-dimensionally arraying a plurality of mirrordevices, each of the mirror devices comprising a mirror which issupported to be pivotable with respect to a mirror substrate, a drivingelectrode which is formed on an electrode substrate facing the mirrorsubstrate, and an antistatic structure which is arranged in a spacebetween the mirror and the electrode substrate.

According to the present invention, there is also provided an opticalswitch characterized by comprising a first mirror array which reflectslight from an input port, and a second mirror array which reflects thelight from the first mirror array and guides the light to an outputport, each of the first mirror array and the second mirror arraycomprising a plurality of above-described mirror devices arrangedtwo-dimensionally.

According to the present invention, there is also provided a method ofmanufacturing a mirror device which includes a mirror substrate having aflat mirror pivotally supported, and an electrode substrate which facesthe mirror substrate and has an electrode to control pivotal movement ofthe mirror, characterized by comprising the first step of preparing themirror substrate having the flat mirror pivotally supported, the secondstep of forming a first metal layer on one surface of the mirror, thethird step of forming a second metal layer on the other surface of themirror, and the fourth step of placing the mirror substrate on theelectrode substrate to make the electrode face the mirror.

According to the present invention, there is also provided a method ofmanufacturing a mirror substrate, characterized by comprising at leastthe first step of preparing an SOI substrate including a substrateportion, a buried insulating layer on the substrate portion, and asilicon layer on the buried insulating layer, the second step of forminga movable portion formation mask pattern on a surface of the siliconlayer and forming the silicon layer by etching using the movable portionformation mask pattern as a mask to form, in a mirror formation regionon the buried insulating layer, a base and a plate-shaped mirrorstructure connected to the base through a pair of connectors, the thirdstep of forming a protective layer that fills spaces between the base,the connectors, and the mirror structure, and the fourth step offorming, on a surface of the substrate portion, a frame formation maskpattern with an opening corresponding to the mirror formation region andremoving the substrate portion and the buried insulating layer byetching using the frame formation mask pattern as a mask to expose thesilicon layer on a side of the substrate portion in the mirror formationregion and form a frame portion outside the mirror formation region.

According to the present invention, there is also provided a method ofmanufacturing a device which includes a mirror substrate having a mirrorpivotally supported, and an electrode substrate which faces the mirrorsubstrate, characterized by comprising the first step of preparing theelectrode substrate having a flat surface, a substantially conicalprojecting portion that projects from the flat surface, and a trenchformed in the flat surface around the projecting portion, the secondstep of forming a metal layer on the flat surface and surfaces of theprojecting portion and trench of the electrode substrate, the third stepof patterning the metal layer while setting focus of an exposureapparatus on the flat surface to form an interconnection on the flatsurface and, at least on the surfaces of the projecting portion andtrench, an electrode connected to the interconnection, and the fourthstep of placing the mirror substrate on the electrode substrate to makethe electrode face the mirror.

Effects of the Invention

An effect of the present invention is to suppress drift of the mirror.Since the antistatic structure is arranged in the space between themirror and the lower substrate, the charge/discharge time constant to apart related to driving of the mirror can be small, or a part with alarge charge/discharge time constant can be eliminated from the vicinityof the driving electrodes.

According to the present invention, since a metal layer is provided notonly on one surface but also on the other surface of the mirror, warp ofthe mirror can be controlled.

According to the present invention, when the base, connectors, andmirror structure are formed on the buried insulating layer, a protectivelayer filling the spaces between them is formed. Even when the buriedinsulating layer in the mirror formation region is removed to expose theboth surfaces of the silicon layer and make the mirror structuremovable, the mirror structure is prevented from moving. As a result,according to the present invention, the mirror structure and connectorsare protected from damage. This allows to form mirror substrates at ahigh yield of non-defective units.

According to the present invention, since a projecting portion is formedin a trench formed in the base to increase the difference of elevationof the projecting portion, the mirror can have a large pivot angle.Interconnections are formed on the base, and the electrodes are formedon the trench formed in the substrate and the projecting portionprojecting from the trench. Hence, setting the focus of the exposureapparatus on the upper surface of the base allows to form theinterconnections and electrodes at a necessary accuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded perspective view of a mirror device according tothe first embodiment of the present invention;

FIG. 2 is a sectional view of the mirror device according to the firstembodiment of the present invention;

FIG. 3 is an equivalent circuit diagram of an electric circuit relatedto driving of a mirror in the mirror device;

FIG. 4 is a sectional view of a mirror device according to the secondembodiment of the present invention;

FIG. 5 is a sectional view of a mirror device according to the thirdembodiment of the present invention;

FIG. 6 is a sectional view of a mirror device according to the fourthembodiment of the present invention;

FIG. 7 is a sectional view of the main part of a mirror device accordingto the fifth embodiment of the present invention;

FIG. 8 is an exploded perspective view showing the structure of a mirrordevice according to the sixth embodiment of the present invention;

FIG. 9 is a sectional view of the main part of the mirror device shownin FIG. 8;

FIG. 10 is a sectional view of a mirror device according to the seventhembodiment of the present invention;

FIG. 11 is a schematic perspective view showing the structures ofconventional mirror substrate and electrode substrate;

FIG. 12 is a schematic perspective view showing the structure of aconventional mirror device;

FIG. 13 is a sectional view of the mirror device shown in FIG. 12;

FIG. 14 is a perspective view showing the structure of the electrodesubstrate of a mirror array according to the eighth embodiment;

FIG. 15 is a schematic perspective view showing the structures of amirror substrate and electrode substrate according to the eighthembodiment;

FIG. 16A is a sectional view taken along a line I-I in FIG. 16B;

FIG. 16B is a plan view of a mirror device according to the eighthembodiment of the present invention;

FIGS. 17A to 17D are views showing a method of manufacturing theelectrode substrate according to the eighth embodiment;

FIGS. 18A to 18C are views showing a method of manufacturing theelectrode substrate according to the eighth embodiment;

FIGS. 19A to 19C are views showing a method of manufacturing theelectrode substrate according to the eighth embodiment;

FIGS. 20A to 20F are views showing a method of manufacturing theelectrode substrate according to the eighth embodiment;

FIG. 21 is a sectional view showing a modification of the electrodesubstrate according to the eighth embodiment;

FIG. 22 is a sectional view showing a modification of the electrodesubstrate according to the eighth embodiment;

FIG. 23 is a schematic view showing a conventional optical switch;

FIG. 24 is a sectional view schematically showing the structure of aconventional mirror array;

FIG. 25A is a schematic sectional view showing the structure of a mirrorarray according to the ninth embodiment;

FIG. 25B is a schematic plan view showing the structure of the mirrorarray according to the ninth embodiment;

FIG. 26 is a schematic sectional view showing the structure of aconventional mirror device;

FIG. 27 is a schematic sectional view showing the structure of a mirrordevice according to the ninth embodiment;

FIG. 28 is a schematic plan view showing the structure of the mirrorarray according to the ninth embodiment;

FIG. 29 is a schematic view showing the structure of an optical switchaccording to the ninth embodiment;

FIG. 30 is a schematic view of the sections of the mirror arraysaccording to the ninth embodiment;

FIG. 31 is a schematic plan view of a mirror device according to the10th embodiment;

FIG. 32 is a schematic view showing the structure of a torsion spring;

FIG. 33A is a schematic view showing the structure of a torsion spring;

FIG. 33B is a graph showing the spring constant of the torsion springshown in FIG. 33A;

FIG. 34A is a schematic view showing the structure of a torsion spring;

FIG. 34B is a graph showing the spring constant of the torsion springshown in FIG. 34A;

FIG. 35 is a schematic sectional view showing the structure of aconventional mirror device;

FIG. 36 is a schematic sectional view showing the structure of themirror device according to the 10th embodiment;

FIG. 37 is a schematic plan view showing the structure of a mirror arrayaccording to the 10th embodiment;

FIG. 38 is a schematic plan view showing modification of a mirrorsubstrate;

FIG. 39A is a schematic sectional view of a mirror device according tothe 11th embodiment;

FIG. 39B is a schematic plan view of the mirror device according to the11th embodiment;

FIG. 40 is a schematic sectional view showing the structure of aconventional mirror device;

FIG. 41 is a schematic sectional view showing the structure of themirror device according to the 11th embodiment;

FIG. 42 is a schematic plan view showing the structure of a mirror arrayaccording to the 11th embodiment;

FIG. 43 is a perspective view showing the structure of a mirror deviceaccording to the 12th embodiment;

FIG. 44 is a schematic sectional view showing the structure of themirror device according to the 12th embodiment;

FIG. 45 is a schematic plan view of a mirror device according to the13th embodiment;

FIG. 46 is a schematic sectional view of a conventional mirror device;

FIG. 47 is a schematic sectional view of the mirror device according tothe 13th embodiment;

FIG. 48 is a schematic plan view of another mirror device according tothe 13th embodiment;

FIG. 49 is a schematic plan view of still another mirror deviceaccording to the 13th embodiment;

FIG. 50 is a schematic plan view of still another mirror deviceaccording to the 13th embodiment;

FIG. 51 is a schematic plan view of a mirror array according to the 13thembodiment;

FIG. 52 is an exploded perspective view showing the structure of amirror device according to the 14th embodiment;

FIG. 53 is a sectional view of the mirror device shown in FIG. 52;

FIG. 54 is a graph showing an actual measurement result of the influenceof an electrostatic force from interconnections on a mirror 1103;

FIG. 55 is a view for explaining the arrangement conditions of themirror 1103, driving electrodes 1003-1 to 1003-4, and interconnectionsin measuring the data shown in FIG. 54;

FIG. 56 is a plan view showing the layout of mirror devices andinterconnections in a mirror array according to the 15th embodiment;

FIG. 57 is an exploded perspective view showing the structure of amirror device according to the 16th embodiment;

FIG. 58 is a sectional view of the mirror device shown in FIG. 57;

FIG. 59 is a view for explaining the principle of suppressing variationsin the tilt angle of a mirror in the 16th embodiment;

FIG. 60 is a sectional view showing the structure of the main part of amirror device according to the 17th embodiment;

FIG. 61 is a sectional view showing the structure of the main part of amirror device according to the 18th embodiment;

FIG. 62 is an exploded perspective view showing the structure of aconventional mirror device;

FIG. 63 is a sectional view of the mirror device shown in FIG. 62;

FIG. 64 is a graph showing an example of a driving voltage vs. tiltangle characteristic curve in the mirror device shown in FIG. 62;

FIG. 65 is a graph showing an example of a driving voltage vs. tiltangle characteristic curve when a mirror sinks and pivots in the mirrordevice shown in FIG. 62;

FIG. 66 is an exploded perspective view showing the structure of amirror device according to the 19th embodiment;

FIG. 67 is a sectional view of the mirror device shown in FIG. 66;

FIG. 68 is a view for explaining a bias voltage application methodaccording to the 19th embodiment;

FIG. 69 is a view for explaining a voltage application method when amirror pivots in the 19th embodiment;

FIG. 70 is a graph showing an example of a driving voltage vs. tiltangle characteristic curve in the mirror device according to the 19thembodiment;

FIG. 71 is a view for explaining the effect of bias voltage applicationto driving electrodes in the 19th embodiment;

FIG. 72 is a view for explaining a driving method in biaxial driving;

FIG. 73 is a view for explaining a bias voltage setting method;

FIG. 74 is a graph showing the relationship between the driving voltageand the tilt angle when the bias voltage changes;

FIG. 75 is a schematic perspective view showing a structural example ofa mirror device according to the 20th embodiment;

FIGS. 76A to 76L are views showing an example of steps in manufacturinga mirror substrate according to the 20th embodiment;

FIG. 77 is a schematic plan view showing a structural example of themirror substrate according to the 20th embodiment;

FIG. 78 is a schematic perspective view partially showing a structuralexample of the mirror substrate according to the 20th embodiment;

FIG. 79 is a schematic perspective view partially showing a structuralexample of the mirror substrate according to the 20th embodiment;

FIG. 80 is a plan view schematically showing a structural example of themirror substrate according to the 20th embodiment;

FIG. 81 is a plan view schematically showing a structural example of themirror substrate according to the 20th embodiment;

FIG. 82 is a sectional view showing the structure of a mirror deviceaccording to the 21st embodiment;

FIG. 83 is a perspective view showing the structure of the mirror deviceaccording to the 21st embodiment;

FIGS. 84A to 84E are views for explaining an example of steps inmanufacturing a mirror substrate included in the mirror device accordingto the 21st embodiment;

FIGS. 85A to 85E are views for explaining an example of steps inmanufacturing the mirror substrate included in the mirror deviceaccording to the 21st embodiment;

FIG. 86 is a sectional view showing a structural example of aconventional mirror device;

FIG. 87 is a sectional view showing a structural example of aconventional mirror device;

FIG. 88 is a plan view for explaining the structure of a mirrorsubstrate according to the 22nd embodiment;

FIG. 89 is a perspective view showing a torsion spring that forms amovable frame connector and a mirror connector;

FIG. 90 is a plan view for explaining the structure of the mirrorsubstrate according to the 22nd embodiment;

FIG. 91 is a perspective view showing a conventional optical switch;

FIG. 92 is a view for explaining a problem of the conventional opticalswitch;

FIG. 93 is a perspective view showing a structure of an optical switchaccording to the 23rd embodiment;

FIG. 94 is an exploded perspective view of a mirror device included inthe optical switch according to the 23rd embodiment;

FIG. 95 is a view showing the relationship between the driving electrodearrangement of a mirror device of an input-side mirror array and thearrangement region of a mirror device of an output-side mirror arrayaccording to the 23rd embodiment;

FIG. 96 is a sectional view of a mirror device according to the 24thembodiment in which driving electrodes are formed on the terrace-shapedprojecting portion of a lower substrate;

FIG. 97 is a sectional view of a mirror device in which drivingelectrodes are formed on the conical projecting portion of a lowersubstrate;

FIG. 98 is a perspective view of the terrace-shaped projecting portion;

FIG. 99 is a perspective view of four driving electrodes divisionallyformed on the terrace-shaped projecting portion;

FIG. 100A is a schematic plan view showing the structure of a mirror ofa mirror device in a mirror array according to the 25th embodiment;

FIG. 100B is a schematic side view of the mirror according to the 25thembodiment;

FIG. 100C is a schematic bottom view of the mirror according to the 25thembodiment;

FIG. 101A is a graph showing the relationship between a gold depositionthickness of a mirror 230 having only an upper surface layer 232 and thewarp amount of the mirror;

FIG. 101B is a graph showing the relationship between a gold depositionthickness of a mirror 230 having only an upper surface layer 232 and thewarp amount of the mirror;

FIG. 102A is a schematic sectional view showing the mirror 230 so as toexplain the warp of the mirror 230;

FIG. 102B is a schematic sectional view showing the mirror 230 so as toexplain the warp of the mirror 230;

FIG. 102C is a schematic sectional view showing the mirror 230 so as toexplain the warp of the mirror 230;

FIGS. 103A to 103E are schematic views for explaining steps inmanufacturing the mirror substrate of a mirror array according to the25th embodiment;

FIGS. 104A to 104C are schematic views for explaining the warp of amirror 230;

FIGS. 105A to 105K are views showing steps in forming a mirror substrateaccording to the 28th embodiment;

FIG. 106 is a perspective view schematically showing the structure of amirror substrate 2300 formed by the steps in FIGS. 105A to 105K;

FIG. 107 is an exploded perspective view showing the structure of aconventional mirror device; and

FIG. 108 is a sectional view of the mirror device shown in FIG. 107.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will be described below indetail with reference to the accompanying drawings.

First Embodiment

A mirror device 1 according to the first embodiment will be describedbelow. As shown in FIGS. 1 and 2, an insulating layer 102 made of asilicon oxide film is formed on a lower substrate 101 of single-crystalsilicon. Four driving electrodes 103-1 to 103-4 are provided on theinsulating layer 102 at the center of the lower substrate 101. Supports104 of single-crystal silicon are provided on both sides of the uppersurface of the lower substrate 101.

In this embodiment, the insulating layer 102 on the surface of eachsupport 104 is partially removed to form a contact hole 106. A metallayer 105 made of, e.g., Au is formed on the contact hole 106.

An upper substrate 151 has an annular gimbal 152 inside. A mirror 153 isprovided inside the gimbal 152. For example, a Ti/Pt/Au layer (notshown) with a three-layered structure is formed on the upper surface ofthe mirror 153. Torsion springs 154 connect the upper substrate 151 tothe gimbal 152 at two 180° opposite points. Similarly, torsion springs155 connect the gimbal 152 to the mirror 153 at two 180° oppositepoints. The X-axis passing through the pair of torsion springs 154 andthe Y-axis passing through the pair of torsion springs 155 intersect ata right angle. As a result, the mirror 153 can pivot around the X- andY-axes each serving as a pivot axis.

In this embodiment, a metal layer 156 made of, e.g., Au and serving asan antistatic structure is formed on the lower surfaces of the uppersubstrate 151, gimbal 152, mirror 153, and torsion springs 154 and 155.

The mirror device generally uses an SOI (Silicon On Insulator) substratecapable of easily obtaining single-crystal silicon and, moreparticularly, a single-crystal silicon plate with a thickness of about10 μm because of the requirements of the surface flatness of the mirror153 and the reliability of the torsion springs 154 and 155. The mirror153 is formed on the SOI substrate. Making the mirror 153 face thedriving electrodes 103-1 to 103-4, the metal layer 105 is bonded to themetal layer 156 by using solder such as an AuSn alloy or a conductiveadhesive such as Ag paste so that the upper substrate 151 bonds to thelower substrate 101.

In this mirror device, the mirror 153 is grounded, A positive ornegative voltage is applied to the driving electrodes 103-1 to 103-4 togenerate an asymmetrical potential difference between the drivingelectrodes 103-1 to 103-4. An electrostatic force attracts the mirror153 and causes it to pivot in an arbitrary direction.

The causes of drift of the mirror will be described next with referenceto FIG. 3 by exemplifying a conventional mirror device shown in FIGS.107 and 108.

Referring to FIG. 3, reference symbol R1 denotes a grounding resistanceof a mirror 8103; R2, a resistance of an insulating layer (not shown inFIGS. 107 and 108) formed on the surface of driving electrodes 8003-1 to8003-4; R3, a grounding resistance of a lower substrate 8001; R4, aninsulating leakage resistance when a leakage current from the drivingelectrodes 8003-1 to 8003-4 and interconnections (not shown in FIGS. 107and 108) to supply a first potential V (V≧0 in this embodiment) to theelectrodes flows to the lower substrate 8001 through an insulating layer8002; C1, a capacitance formed between the mirror 8103 and the drivingelectrodes 8003-1 to 8003-4; C2, a capacitance of an insulating layerformed on the surfaces of the driving electrodes 8003-1 to 8003-4; C3,an interconnection stray capacitance (the capacitance of the insulatinglayer 8002) formed between the lower substrate 8001 and the drivingelectrodes 8003-1 to 8003-4 and interconnections to supply the firstpotential to the electrodes; and REG, a power supply which applies thefirst potential to the driving electrodes 8003-1 to 8003-4 through theinterconnections and also applies a second potential (the secondpotential is equal to or different from the first potential, and in thisembodiment, a ground potential) to the mirror 8103 and lower substrate8001.

The drift of the mirror 8103 can roughly be classified into two types.Drift of the first type occurs when the voltage between the mirror 8103and the driving electrodes 8003-1 to 8003-4 does not follow the voltageapplied to the electrical interconnections because the interconnectionsto apply the voltage between the mirror 8103 and the driving electrodes8003-1 to 8003-4 are imperfect. Drift of the second type occurs when anelectrically stray part with an indefinite potential polarizes uponvoltage application, causes gradual electrification due to some reason,or gradually loses accumulated charges to influence the driving force ofthe mirror 8103. Another example of the part where such polarization orelectrification occurs is a part connected to the first potential orsecond potential at a high resistance.

That is, the drift fundamentally occurs when the charge/discharge timeconstant in parts (the mirror 8103, the driving electrodes 8003-1 to8003-4, and the structures near the driving electrodes 8003-1 to 8003-4)related to driving of the mirror 8103 is large. Two measures areavailable to suppress the drift. One measure is to reduce thecharge/discharge time constant. The other is to eliminate the parts withthe large charge/discharge time constant from the vicinity of thedriving electrodes 8003-1 to 8003-4.

In the mirror device that deflects a light beam by making the mirror8103 pivot by an electrostatic force, a slight displacement of the tiltangle of the mirror 8103 is amplified to a displacement of the lightbeam projection point. Hence, it is necessary to minimize the drift ofthe mirror 8103. Especially in a spatial optical switch using the mirrordevice, a displacement of the tilt angle of the mirror 8103 changes toan insertion loss variation. Hence, the optical switch is notpractically usable if drift occurs.

In this embodiment, an antistatic structure is formed on parts (themirror 8103, the driving electrodes 8003-1 to 8003-4, and the structuresnear the driving electrodes 8003-1 to 8003-4) related to driving of themirror 8103 on the basis of the two measures.

As described above, the upper substrate 151, gimbal 152, mirror 153, andtorsion springs 154 and 155 are integrally made of single-crystalsilicon of the SOI substrate. A ground potential is applied to themirror 153 through the upper substrate 151, torsion springs 154, gimbal152, and torsion springs 155. However, the actual potential of themirror 153 is the potential of a point A in FIG. 3, which is higher thanthe ground potential before the capacitance C1 formed between the mirror153 and the driving electrodes 103-1 to 103-4 finishes charge. This isbecause the grounding resistance R1 of the mirror 153 (silicon layer)shown in FIG. 3 exists.

To prevent drift, it is necessary to reliably ensure electricalconnection to fix the potential of the lower surface of the mirror 153facing the driving electrodes 103-1 to 103-4. However, it is generallynot easy to attain it. The silicon layer serving as the mirror 153 iselectrically disconnected from the base silicon layer of the SOIsubstrate by an insulating layer. To obtain a light beam reflectingfunction, a metal layer of, e.g., Au is deposited on the upper surfaceof the mirror 153. It is however normally impossible to expect thedeposited metal layer to electrical connect the silicon layer of themirror 153 to the base silicon layer of the SOI substrate. A nativeoxide film, i.e., a silica layer with insulating properties is usuallyformed on the silicon surface. For this reason, even when the metallayer deposited on the upper surface of the silicon layer serving as themirror 153 electrically connects to a potential, the silicon layer ofthe mirror 153 itself does not always connect to that potential.

To fix the potential of the lower surface of the mirror 153 facing thedriving electrodes 103-1 to 103-4, it is effective to ensure electricalcontact to the lower surface of the mirror 153 directly from the sidefacing the driving electrodes 103-1 to 103-4.

In this embodiment, as shown in FIGS. 1 and 2, the metal layer 156 madeof, e.g., Au and serving as an antistatic structure is formed on thelower surfaces of the upper substrate 151, gimbal 152, mirror 153, andtorsion springs 154 and 155. A second potential is applied to the metallayer 156. This structure reduces the grounding resistance R1 in FIG. 3.

The upper substrate 151, gimbal 152, mirror 153, and torsion springs 154and 155 are integrally made of single-crystal silicon. Hence, when themetal layer 156 is formed on the lower surfaces, and the secondpotential is applied from an end of the metal layer 156, the lowersurface of the mirror 153 is fixed to the second potential. However, itis sometimes difficult to directly apply the second potential from aside surface of the mirror device to the metal layer 156.

In this embodiment, the second potential is applied to the metal layer156 through the lower substrate 101 and supports 104. To obtain thiselectrical connection, in this embodiment, the insulating layer 102 onthe surface of each support 104 is partially removed to form the contacthole 106. The metal layer 105 made of, e.g., Au is formed on the contacthole 106. The metal layer 105 connects to the metal layer 156 on theside of the upper substrate 151. It is therefore easy to ensureelectrical connection to the metal layer 156 that has difficulty ininterconnection.

The metal layer 156 serving as an antistatic structure is formed on thelower surface of the 153 facing the driving electrodes 103-1 to 103-4,and the second potential is applied to the metal layer 156, determiningthe potential of the lower surface of the mirror 153. This allows tosuppress drift of the mirror 153.

Second Embodiment

The second embodiment of the present invention will be described nextwith reference to FIG. 4. The same reference numerals as in FIGS. 1 and2 denote the same parts in FIG. 4. In the first embodiment, the secondpotential is applied to the metal layer 156 through the lower substrate101 and supports 104, which are made of single-crystal silicon. Instead,the second potential may be applied to a metal layer 156 through metalsupports 107 formed on an insulating layer 102, as shown in FIG. 4. Toform the supports 107, a metal such as Au is deposited by, e.g. plating.In the second embodiment, since it is possible to ensure electricalconnection to the metal layer 156 without intervening a silicon layer,the potential of the lower surface of a mirror 153 can properly be set.

Third Embodiment

The third embodiment of the present invention will be described nextwith reference to FIG. 5. The same reference numerals as in FIGS. 1 and2 denote the same parts in FIG. 5. In the first embodiment, the secondpotential is applied to the metal layer 156 through the lower substrate101 and supports 104, which are made of single-crystal silicon. Instead,an insulating layer 102 on a lower substrate 101 may partially beremoved to form contact holes 109, as shown in FIG. 5. Supports 108 madeof, e.g., Au may be formed on the contact holes 109 to apply the secondpotential to a metal layer 156 through the supports 108. Thisfacilitates electrical connection to the supports 108.

In the first to third embodiments, to obtain reliable electricalconnection to the metal layer 156, an oxide film on the surface of themetal layer 105 or 156 or the supports 107 and 108 may be removed by,e.g., an acid. Not only mechanical contact but also solder such as anAuSn alloy or a conductive adhesive such as Ag paste can ensureelectrical connection between the metal layers 105 and 156 or electricalconnection between the metal layer 156 and the supports 107 or 108.

Fourth Embodiment

The fourth embodiment of the present invention will be described nextwith reference to FIG. 6. The same reference numerals as in FIGS. 1 and2 denote the same parts in FIG. 6. In this embodiment, an antistaticstructure exists on the side of a tower substrate 101. Since the firstpotential is supplied to driving electrodes 103-1 to 103-4 normallythrough a metal interconnection, the potential of the driving electrodes103-1 to 103-4 is never indeterminate. Hence, the problem that thevoltage of the driving electrodes 103-1 to 103-4 does not follow thevoltage applied to the interconnection is avoidable.

Electrically stray parts with an indeterminate potential can beclassified into several parts. An example of such parts is the lowersubstrate 101. Normally, a mirror 153 has a larger area than the counterdriving electrodes 103-1 to 103-4. For this reason, the lower substrateitself may exist at positions facing the mirror 153. If the potential ofthe lower substrate 101 is different from that of the mirror 153, anelectrostatic force corresponding to charges present in the lowersubstrate 101 facing the mirror 153 acts on the mirror 153. The chargesgradually move to the first potential side through a resistance R4 inFIG. 3 or to the side of the second potential through a resistance R3.Hence, the mirror 153 drifts.

To prevent the mirror 153 from drifting due to the lower substrate 101,it is important to set the lower substrate 101 to the second potentialequal to the mirror 153. In this embodiment, the antistatic structure isformed in the following way. The lower substrate 101 uses a conductivematerial (single-crystal silicon in this embodiment). An oxide film onthe lower or side surfaces of the lower substrate 101 is removed. Ametal layer 110 is deposited at portions without the oxide film. Thesecond potential is applied to the lower substrate 101 through the metallayer 110. This structure decreases the grounding resistance R3 in FIG.3.

As described above, in this embodiment, the lower substrate 101 uses aconductive material. The oxide film on the lower or side surfaces of thelower substrate 101 is removed. The metal layer 110 is deposited atportions without the oxide film. The second potential is applied to thelower substrate 101 through the metal layer 110. Hence, it is possibleto determine the lower substrate 101 to the same potential as that ofthe mirror 153 and suppress drift of the mirror 153.

Fifth Embodiment

The fifth embodiment of the present invention will be described nextwith reference to FIG. 7. The same reference numerals as in FIGS. 1 and2 denote the same parts in FIG. 7.

The fourth embodiment ensures reliable electrical connection of thelower substrate 101. The next problem is insulating layers present onthe surfaces of driving electrodes 103-1 to 103-4. Generally, aninsulating layer is formed on the surface of each of the drivingelectrodes 103-1 to 103-4 to protect the electrode and prevent a shortcircuit, although not illustrated in FIGS. 1, 2, 4, 5, 107, and 108. Theinsulating layers polarize upon application of the first potential tothe driving electrodes 103-1 to 103-4. Even the insulating layers have afinite electrical conductivity although it is very low. Hence, chargesmove at a predetermined time constant and finally make the insulatinglayers equipotential to the driving electrodes 103-1 to 103-4.

Finally, the distance between the driving electrodes 103-1 to 103-4 andthe mirror 153 decreases by an amount corresponding to the thickness ofthe insulating layers. The force between the driving electrodes 103-1 to103-4 and the mirror 153 that attract each other increases as thedistance therebetween decreases. For this reason, the mirror 153 drifts.The discharge time constant of polarization in the insulating layers isnormally large and ranges from several min to several hrs.

In this embodiment, an insulating layer 111 on the surface of each ofthe driving electrodes 103-1 to 103-4 has an opening 112 serving as theabove-described antistatic structure, as shown in FIG. 7. This indicatesremoval (short circuit) of the capacitance C1 and resistance R2 in FIG.3.

As described above, in this embodiment, the insulating layer 111 on thesurface of each of the driving electrodes 103-1 to 103-4 has the opening112. It is therefore possible to eliminate a portion with a largecharge/discharge time constant from the vicinity of the drivingelectrodes 103-1 to 103-4 and prevent drift of the mirror 153.

Sixth Embodiment

The sixth embodiment of the present invention will be described nextwith reference to FIGS. 8 and 9. The same reference numerals as in FIGS.1 and 2 denote the same parts in FIGS. 8 and 9.

Referring to FIGS. 8 and 9, interconnections 113-1 to 113-4 supply afirst potential to driving electrodes 103-1 to 103-4, respectively.Metal layers 114 are formed around the driving electrodes 103-1 to103-4.

The fifth embodiment allows to avoid polarization/discharge in theinsulating layers on the driving electrodes 103-1 to 103-4. The nextproblem is an insulating layer 102 on the surface of a lower substrate101. It is easy to remove the insulating layers on the drivingelectrodes 103-1 to 103-4, as in the fifth embodiment. However, it isoften impossible to remove the insulating layer 102 because the drivingelectrodes 103-1 to 103-4 and interconnections are formed on theinsulating layer 102.

In this embodiment, the metal layers 114 serving as the antistaticstructure are formed around the driving electrodes 103-1 to 103-4 on theinsulating layer 102. A second potential is applied to the metal layers114, like a mirror 153. As described above, the area of the mirror 153is larger than that of the driving electrodes 103-1 to 103-4. For thisreason, the insulating layer 102 around the driving electrodes 103-1 to103-4 faces the outer peripheral portion of the mirror 153. The metallayers 114 are formed on the insulating layer 102 facing the mirror 153.The metal layers 114 can be formed simultaneously together with thedriving electrodes 103-1 to 103-4 and interconnections 113-1 to 113-4.

To apply the second potential to the plurality of metal layers 114, itis necessary to provide interconnections connected to the metal layers114. However, this is not practical because the number ofinterconnections increases, and the interconnections must cross over theinterconnections 113-1 to 113-4 to the driving electrodes 103-1 to103-4. As shown in FIG. 9, the insulating layer 102 on the lowersubstrate 101 is partially removed to form contact holes 115. When themetal layers 114 are formed on the contact holes 115, the metal layers114 are equipotential to the lower substrate 101. This structurefacilitates electrical connection to the metal layers 114 withoutinterconnection routing on the surface layer.

As described above, in this embodiment, the metal layers 114 serving asthe antistatic structure are formed around the driving electrodes 103-1to 103-4 on the insulating layer 102, and the second potential isapplied to the metal layers 114. It is possible to eliminate a portionwith a large charge/discharge time constant from the vicinity of thedriving electrodes 103-1 to 103-4 and prevent drift of the mirror 153.

Seventh Embodiment

The seventh embodiment of the present invention will be described nextwith reference to FIG. 10. The same reference numerals as in FIGS. 1, 2,8, and 9 denote the same parts in FIG. 10.

In the sixth embodiment, the interconnections 113-1 to 113-4 are formedon the same plane as the driving electrodes 103-1 to 103-4. As shown inFIG. 10, interconnections 113-1 to 113-4 and driving electrodes 103-1 to103-4 may be formed on different planes. In this embodiment, theinterconnections 113-1 to 113-4 are formed on an insulating layer 102.An insulating layer 115 is deposited on the insulating layer 102 andinterconnections 113-1 to 113-4. The driving electrodes 103-1 to 103-4and metal layers 114 are formed on the insulating layer 115.Interconnections 116 connected to the metal layers 114 are formed on theinsulating layer 102 simultaneously together with the interconnections113-1 to 113-4. The metal layers 114 receive the same second potentialas that of a mirror 153 or the second potential with an offset throughthe interconnections 116. This structure facilitates electricalconnection to the metal layers 114 without interconnection routing onthe surface layer.

In this embodiment, interconnections 117 may also be formed on theinsulating layer 102 simultaneously together with the interconnections113-1 to 113-4. Metal supports 118 to support an upper substrate 153 maybe formed on the insulating layer 115 to which one end of eachinterconnection 117 is exposed. At this time, the driving electrodes103-1 to 103-4 are spaced apart from the supports 118 by a distance(e.g., about several μm to 25 μm) not to cause discharge to the metallayers 114 and supports 118. To form the supports 118, a metal such asAu is deposited by, e.g., plating. This structure allows to apply thesecond potential to a metal layer 156 through the supports 118 andinterconnections 117 and ensure potential setting on the lower surfaceof the mirror 153.

In this embodiment, the metal layer 156 may receive the second potentialthrough a tower substrate 101 and supports 104, as in the firstembodiment, instead of using the supports 118. In this case, as wellshown in FIG. 2, the insulating layer 102 on the surface of each support104 is partially removed to form a contact hole 106. A metal layer 105made of, e.g., Au is formed on the contact hole 106. The metal layer 105connects to the metal layer 156 on the side of the upper substrate 151.This structure also allows to apply the second potential to the metallayer 156 and ensure potential setting on the lower surface of themirror 153.

As described above, in this embodiment, the metal layers 114 serving asthe antistatic structure are formed around the driving electrodes 103-1to 103-4 on the insulating layer 102, and the second potential isapplied to the metal layers 114 through the interconnections 116. It ispossible to eliminate a portion with a large charge/discharge timeconstant from the vicinity of the driving electrodes 103-1 to 103-4 andprevent drift of the mirror 153.

Referring to FIG. 10, the interconnections 116 connected to the metallayers 114 and the interconnections 117 connected to the supports 118are separately provided. However, the interconnections may connect toeach other.

in the first to seventh embodiments, first potential≧second potential.However, the present invention is not limited to this, and firstpotential≦second potential may also hold.

Eighth Embodiment

The eighth embodiment of the present invention will be described next.

FIGS. 11 and 12 show an example of a conventional mirror array. FIGS. 11and 12 mainly partially illustrate a mirror device having a mirror as aconstituent unit of a array. A mirror array includes a plurality ofmirror devices two-dimensional arranged in a matrix.

A mirror device 8200 has a structure in which a mirror substrate 8201with a mirror and an electrode substrate 8301 with electrodes arearranged in parallel.

The mirror substrate 8201 has a frame portion 8210, a movable frame 8220arranged in an opening of the frame portion 8210 by a pair of movableframe connectors 8211 a and 8211 b, and a mirror 8230 that has an almostcircular shape when viewed from the upper side and arranged in anopening of the movable frame 8220 by a pair of mirror connectors 8221 aand 8221 b. A frame-shaped member 8240 surrounding the movable frame8220 and mirror 8230 is formed on the upper surface of the frame portion8210.

The pair of movable frame connectors 8211 a and 8211 b including zigzagtorsion springs and provided in the notches of the movable frame 8220connect the frame portion 8210 to the movable frame 8220. The pair ofconnectors 8221 a and 8221 b including zigzag torsion springs andprovided in the notches of the movable frame 8220 connect the movableframe 8220 to the mirror 8230.

The electrode substrate 8301 has a plate-shaped base 8310 and aprojecting portion 8320 projecting from the surface (upper surface) ofthe base 8310. The projecting portion 8320 includes a third terrace 8323formed on the upper surface of the base 8310, a second terrace 8322formed on the upper surface of the third terrace 8323, a first terrace8321 formed on the upper surface of the second terrace 8322, and a pivot8330 formed on the upper surface of the first terrace 8321.

Four sector electrodes 8340 a to 8340 d are formed on the upper surfaceof the base 8310 including the outer surface of the projecting portion8320. Around the electrodes 8340 a to 8340 d, concave portions 8350 a to8350 d that have an almost rectangular shape when viewed from the upperside are formed at positions facing the movable frame connectors 8211 aand 8211 b and mirror connectors 8221 a and 8221 b of the counter mirrorsubstrate 8201. A pair of convex portions 8360 a and 8360 b are formedon the upper surface of the base 8310 to sandwich the first to thirdterraces 8321 to 8323 and the concave portions 8350 a to 8350 d.Interconnections 8370 are formed on the upper surface of the base 8310between the concave portion 8350 a and the convex portion 8360 a andbetween the concave portion 8350 c and the convex portion 8360 b. Theelectrodes 8340 a to 8340 d connect to the interconnections 8370 throughleads 8341 a to 8341 d.

The above-described mirror substrate 8201 and electrode substrate 8301form the mirror device 8200 shown in FIG. 12 by joining the lowersurface of the frame portion 8210 to the upper surfaces of the convexportions 8360 a and 8360 b such that the mirror 8230 faces theelectrodes 8340 a to 8340 d corresponding to it. A mirror array having aplurality of mirror devices 8200 arranged in a matrix is manufactured bya method to be described below.

The mirror substrate 8201 is formed from an SOI (Silicon On Insulator)substrate.

First, a side (major surface: SOI layer) of the SOI substrate with aburied insulating layer 8241 undergoes known photolithography andetching such as DEEP RIE to form, in the single-crystal silicon layer,trenches conforming to the shapes of the frame portion 8210, movableframe connectors 8211 a and 8211 b, movable frame 8220, mirrorconnectors 8221 a and 8221 b, and mirror 8230.

A resist pattern with openings in predetermined regions corresponding tothe trenches is formed on the lower surface of the SOT substrate. Thesilicon is selectively etched from the lower surface of the SOIsubstrate by using an etchant such as potassium hydroxide. In thisetching, the opening and frame-shaped member 8240 are formed on thelower surface of the SOI substrate by using the buried insulating layer8241 as an etching stopper layer.

A region of the buried insulating layer 8241 exposed to the opening isremoved by wet etching using hydrofluoric acid or dry etching using aCF-based gas.

With this process, the mirror substrate 8201 having the above-describedshape is formed.

On the other hand, the electrode substrate 8301 is formed from, e.g., asilicon substrate.

First, a silicon substrate is selectively etched by using, as a mask, apredetermined mask pattern made of a silicon nitride film or siliconoxide film and an alkaline solution such as a potassium hydroxidesolution. The base 8310, first to third terraces 8321 to 8323, pivot8330, concave portions 8350, and convex portions 8360 a and 8360 b areformed by repeating the above-described process.

The surface of the silicon substrate on the etched side is oxidized toform a silicon oxide film.

A metal film is formed on the silicon oxide film by, e.g., vapordeposition and patterned by known photolithography and etching to formthe electrodes 8340 a to 8340 d, leads 8341 a to 8341 d, andinterconnections 8370.

With this process, the electrode substrate 8301 having theabove-described shape is formed.

Then, the mirror substrate 8201 is bonded to the electrode substrate8301 to form a mirror array having the mirror device 8200 that can movethe mirror 8230 by applying an electric field to the electrodes 8340 ato 8340 d. To improve the reflectance of the mirror 8230, a metal filmsuch as a gold film may be formed on the upper surface of the mirror8230.

In the mirror device 8200 of the mirror array, an electric fieldgenerated by applying individual voltages to the electrodes 8340 a to8340 d through the interconnections 8370 gives an attracting force tothe mirror 8230 and causes it to pivot by an angle of several degrees.The pivot operation of the mirror 8230 will be described with referenceto FIG. 13. For the descriptive convenience, the vertical direction ofFIG. 13 viewed from the front side will be called a height or depthdirection. The upper part of FIG. 13 will be called an upper side, andthe lower part of FIG. 13 will be called a lower side.

As a characteristic feature of an optical MEMS mirror device, the mirrorcan stably statically pivot to only an angle that is uniquely decided bythe electrode structure of the electrode substrate. As shown in FIG. 13,a pull-in pivot angle θ_(p) of the mirror 8230 and a tilt angle θ of theelectrodes 8340 a to 8340 d formed on the projecting portion 8320 have arelationship approximately given byθ_(p)=(⅓)θ  (1)

To make the mirror 8230 pivot largely at a low voltage as much aspossible, it is necessary to arrange the electrodes 8340 a to 8340 dfacing the mirror 8230 such that they can have the same area as that ofthe mirror 8230 and also to increase the tilt angle of the projectingportion 8320 on which the electrodes 8340 a to 8340 d are formed. Thiscan be achieved by, e.g., increasing the difference of elevation of theprojecting portion 8320.

However, it is conventionally difficult to increase the difference ofelevation of the projecting portion 8320 because the interconnections8370 are formed on the same surface as the bottom surface of theprojecting portion 8320.

As described above, the interconnections 8370 on the electrode substrate8301 are formed by applying a photoresist to a metal film, transferringthe pattern of the interconnections 8370 to the photoresist by anexposure apparatus, and executing etching. The depth (depth of field) towhich the exposure apparatus can precisely transfer a pattern is limited(50 to 70 μm at maximum). If exposure is done beyond this limitation,the pattern to be transferred to the photoresist defocuses so that it isdifficult to form a fine interconnection pattern. Especially, since theinterconnections 8370 include a lot of interconnections, interconnectionpattern formation at an accuracy of several μm is required.

Conventionally, as shown in FIGS. 11 to 13, the interconnections 8370are formed on the base 8310, i.e., on the bottom layer of the thirdterrace 8323 of the projecting portion 8320. Since exposure is done bysetting the lower limit of focus of the exposure apparatus to theinterconnections 8370, the difference of elevation of the projectingportion 8320 must be limited to 50 to 70 μm. However, to increase thepivot angle of the mirror 8230, the difference of elevation of theprojecting portion 8320 must be more than the above-described limitationand, preferably, 100 μm or more. To attain this difference of elevation,it is necessary to use a special exposure apparatus with a large focusrange or execute exposure a plurality of number of times for therespective elevations, resulting in an increase in the cost.

This embodiment has been made to solve the above-described problem andhas as its object to provide a mirror device capable of achievinglow-voltage driving and cost reduction and increasing the pivot angle ofa mirror, a mirror array, and a mirror device manufacturing method.

This embodiment will be described next with reference to FIGS. 14 to16B. FIGS. 14 to 16B mainly partially illustrate a mirror device havinga mirror as a constituent unit of a mirror array. A mirror arrayaccording to this embodiment includes a plurality of mirror devicestwo-dimensionally formed in a matrix.

A mirror device according to this embodiment has a structure in which amirror substrate 200 with a mirror and an electrode substrate 300 withelectrodes are arranged in parallel.

The mirror substrate 200 has a plate-shaped frame portion 210 with anopening having an almost circular shape when viewed from the upper side,a movable frame 220 with an opening having an almost circular shape whenviewed from the upper side and arranged in the opening of the frameportion 210 by a pair of movable frame connectors 211 a and 211 b, and amirror 230 having an almost circular shape when viewed from the upperside and arranged in the opening of the movable frame 220 by a pair ofmirror connectors 221 a and 221 b. A frame-shaped member 240 surroundingthe movable frame 220 and mirror 230 is formed on the upper surface ofthe frame portion 210.

The pair of movable frame connectors 211 a and 211 b including zigzagtorsion springs and provided in the notches of the movable frame 220connect the frame portion 210 to the movable frame 220. This structuremakes the movable frame 220 pivotable about a pivot axis (movable framepivot axis) passing through the pair of movable frame connectors 211 aand 211 b.

The pair of mirror connectors 221 a and 221 b including zigzag torsionsprings and provided in the notches of the movable frame 220 connect themovable frame 220 to the mirror 230. This structure makes the mirror 230pivotable about a pivot axis (mirror pivot axis) passing through thepair of mirror connectors 221 a and 221 b.

The movable frame pivot axis and mirror pivot axis intersect each otherat a right angle.

The electrode substrate 300 has a plate-shaped base 320, an outer trench330 formed on the base 320 and having an almost rectangular shape whenviewed from the upper side, and a projecting portion 340 formed in theouter trench 330 and having an almost conical shape. The surface of thebase 320 with the outer trench 330 and projecting portion 340 has aninsulating film 321.

Four sector electrodes 360 a to 360 d are formed on the outer surface ofthe projecting portion 340 and the upper surface of the outer trench 330to form a circle that is concentric to the mirror 230 and has the samearea as the mirror 230. A pair of convex portions 370 a and 370 b areformed on an upper surface 320 a of the base 320 to sandwich the outertrench 330. Interconnections 380 are formed on the upper surface 320 aof the base 320 between the convex portions 370 a and 370 b and theouter trench 330. The interconnections 380 connect to the electrodes 360a to 360 d through leads 361 a to 361 d.

The outer trench 330 includes a concave portion formed in the surface ofthe base 320. The concave portion has a truncated pyramidal shape withan opening (upper surface) larger than the bottom surface portion. Theprojecting portion 340 is formed on the surface of the outer trench 330.When the projecting portion 340 is formed not on the base 320 but on theouter trench 330 formed in the surface of the base 320, the differenceof elevation of the projecting portion 340 can increase.

The electrodes 360 a to 360 d are formed on the outer trench 330 andprojecting portion 340.

The projecting portion 340 includes a third terrace 343 formed on thesurface (bottom surface) of the outer trench 330 and having a truncatedpyramidal shape, a second terrace 342 formed on the upper surface of thethird terrace 343 and having a truncated pyramidal shape, a firstterrace 341 formed on the upper surface of the second terrace 342 andhaving a truncated pyramidal shape, and a pivot 350 formed on the uppersurface of the first terrace 341 and having a truncated pyramidal shape.

As shown in FIG. 14, the tower surface of the second terrace 342, i.e.,the upper surface of the third terrace 343 is flush with the uppersurface 320 a of the base 320.

A mirror array having a plurality of mirror devices two-dimensionallyarranged in a matrix is formed by joining the mirror substrate 200 tothe electrode substrate 300 with the above-described structure and, morespecifically, joining the upper surfaces of the convex portions 370 aand 370 b of the electrode substrate 300 to the lower surface of theframe portion 210 of the mirror substrate 200 such that the mirror 230of the mirror substrate 200 faces the electrodes 360 a to 360 dcorresponding to the mirror 230.

A method of manufacturing the electrode substrate 300 will be describednext with reference to FIGS. 17A to 20F.

First, to form the pivot 350 on the silicon substrate 300, a siliconsubstrate 400 is oxidized to form an insulating film 401 made of SiO₂ onthe surface of the silicon substrate 400, as shown in FIG. 17A.

As shown in FIG. 17B, a photoresist material is applied to the uppersurface of the insulating film 401 to form a protective film 402.

As shown in FIG. 17C, the protective film 402 is patterned by knownphotolithography to form a mask pattern (pivot preformation maskpattern) 403.

As shown in FIG. 17D, the insulating film 401 is etched by using thepivot preformation mask pattern 403 as a mask to form a mask pattern(pivot formation mask) 404 in the insulating film 401. This etching canbe done by, e.g., known wet etching or dry etching.

As shown in FIG. 18A, the pivot preformation mask pattern 403 is removedby, e.g., ashing.

As shown in FIG. 18B, the silicon substrate 400 is etched by using thepivot formation mask pattern 404 as a mask to form the pivot 350. Thisetching is done by wet etching using an alkaline solution such as apotassium hydroxide solution.

As shown in FIG. 18C, the pivot formation mask pattern 404 is removedfrom the upper surface of the silicon substrate 400 by, e.g.,hydrofluoric acid. The pivot 350 is thus formed on the surface of thesilicon substrate 400.

The first terrace 341 is formed on the surface of the silicon substrate400, as shown in FIG. 19A, in accordance with the same procedures as inFIGS. 17A to 18C.

The second terrace 342 is formed on the surface of the silicon substrate400, as shown in FIG. 19B, in accordance with the same procedures informing the first terrace 341.

The outer trench 330, third terrace 343, and convex portions 370 a and370 h are formed on the surface of the silicon substrate 400, as shownin FIG. 19C, in accordance with the same procedures as in forming thesecond terrace 342. The shape of the base 320 is thus formed.

The pivot 350 is formed such that its difference of elevation, i.e., thedistance from the lower surface of the third terrace 343 to the end ofthe pivot 350 obtains a desired value of, e.g., 100 μm. The uppersurface of the third terrace 343 is flush with the upper surface 320 aof the base 320.

Next, as shown in FIG. 20A, the base 320 is oxidized to form theinsulating film 321 on the surface of the base 320 with the projectingportion 340.

As shown in FIG. 20B, a metal film (metal film forelectrodes/interconnections) 405 made of, e.g., A1 is formed on thesurface of the base 320 with the insulating film 321 by, e.g.,sputtering or vapor deposition.

As shown in FIG. 20C, a protective film 406 made of a photoresistmaterial is formed on the surface of the metal film 405.

As shown in FIG. 20D, the protective film 406 is patterned by knownphotolithography to form a pattern 407 with the interconnection patternsof the electrodes 360 a to 360 d, leads 361 a to 361 d, andinterconnections 380 shown in FIGS. 14, 16A, and 16B.

The focus of the exposure apparatus to transfer the interconnectionpattern to the protective film 406 is set to the upper surface 320 a ofthe base 320 on which the interconnections 380 are to be formed. Thisallows to form the interconnections 380 at the highest resolution.

On the other hand, the electrodes 360 a to 360 d and leads 361 a to 361d are located, in the height direction (vertical direction when FIG. 16Ais viewed from the front side), between the first terrace 341 and theouter trench 330 formed in the upper surface 320 a of the base 320,i.e., from the upper part to the lower part of the upper surface 320 aof the base 320 while almost centered on the upper surface 320 a. Forthis reason, when the exposure apparatus executes exposure with thefocus on the upper surface 320 a of the base 320, pattern transfer isdone even for the electrodes 360 a to 360 d and leads 361 a to 361 d ina region with relatively small image defocus within or near the rangewhere the exposure apparatus can precisely transfer the pattern. It istherefore possible to accurately form the electrodes 360 a to 360 d andleads 361 a to 361 d.

As shown in FIG. 20E, the metal film 405 is etched by using the pattern407 as a mask to form an interconnection layer 408 including theelectrodes 360 a to 360 d, leads 361 a to 361 d, and interconnections380 shown in FIGS. 14 to 16B.

As shown in FIG. 20F, the pattern 407 is removed by, e.g., ashing.

The mirror array according to this embodiment which has the thusmanufactured electrode substrate 300 can make the mirror 230 pivot byapplying a predetermined bias voltage to all the electrodes 360 a to 360d through the interconnections 380 and applying individual displacementvoltages to the electrodes 360 a to 360 d.

According to this embodiment, even when the conventional exposureapparatus is used, the difference of elevation of the projecting portion340 can have a desired value of, e.g., about 100 μm. Hence, theprojecting portion 340 can have a large tilt angle so that the mirror230 can have a pivot angle larger than before. In addition, theelectrodes 360 a to 360 d have the same size as the mirror 230 and cantherefore drive the mirror 230 at a low voltage.

The outer trench need not always have the above-described single-stagestructure but may have a multistage structure. FIG. 21 shows a mirrorarray having an outer trench with a multistage structure. The same namesand reference numerals as in the mirror device shown in FIGS. 14 to 16Bdenote the same constituent elements in FIG. 21, and a descriptionthereof will be omitted as needed. FIG. 21 is a sectional view showingthe same section as in FIGS. 16A and 16B.

An electrode substrate 301 shown in FIG. 21 has the plate-shaped base320, a first outer trench 331 including a concave portion formed on thebase 320 and having an almost rectangular shape when viewed from theupper side, a second outer trench 322 including a concave portion formedon the first outer trench 331 and having an almost rectangular shapewhen viewed from the upper side, and the almost conical projectingportion 340 formed on the second outer trench 332. The insulating film321 is formed on the surface of the base 320 with the first outer trench331, second outer trench 332, and projecting portion 340. The foursector electrodes 360 a to 360 d are formed on the outer surface of theprojecting portion 340 and the upper surface of the second outer trench332 to form a circle that is concentric to the mirror 230. The pair ofconvex portions 370 a and 370 b (not shown) are formed on an uppersurface 320 a of the base 320 to sandwich the first outer trench 331.The interconnections 380 are formed on the base 320 between the convexportions 370 a and 370 b and the first outer trench 331. Theinterconnections 380 connect to the electrodes 360 a to 360 d throughthe leads 361 a to 361 d.

The first outer trench 331 includes a concave portion formed in the base320. The concave portion has a truncated pyramidal shape with an uppersurface larger than the bottom surface. The upper surface of the firstouter trench 1331 is flush with the upper surface of the second terrace.

The second outer trench 332 includes a concave portion formed in thebottom surface of the first outer trench 331. The concave portion has atruncated pyramidal shape with an upper surface larger than the bottomsurface. The projecting portion 340 is formed on the bottom surface ofthe second outer trench 332. The electrodes 360 a to 360 d are formed onthe second outer trench 332 and projecting portion 340.

The projecting portion 340 includes the third terrace 343 formed on thesurface (bottom surface) of the second outer trench 332 and having atruncated pyramidal shape, the second terrace 342 formed on the uppersurface of the third terrace 343 and having a truncated pyramidal shape,the first terrace 341 formed on the upper surface of the second terrace342 and having a truncated pyramidal shape, and the pivot 350 formed onthe upper surface of the first terrace 341 and having a truncatedpyramidal shape.

As shown in FIG. 21, the lower surface of the first terrace 341, i.e.,the upper surface of the second terrace 342 is flush with the surface ofthe base 320. The lower surface of the second terrace 342, i.e., theupper surface of the third terrace is flush with the upper surface ofthe first outer trench 331.

The projecting portion 340 is formed such that its difference ofelevation obtains a desired value of, e.g., about 100 μm.

Even in FIG. 21, in transferring a pattern serving as the electrodes 360a to 360 d, leads 361 a to 361 d, and interconnections 380 to aphotoresist, the exposure apparatus sets the focus on the upper surfaceof the base 320. Hence, the interconnections 380 can be formed at anaccuracy of several μm. Even for the electrodes 360 a to 360 d and leads361 a to 361 d, pattern transfer is done in a region with relativelysmall image defocus within or near the range where the exposureapparatus can precisely transfer the pattern. It is therefore possibleto accurately form the electrodes 360 a to 360 d and leads 361 a to 361d.

As a result, the difference of elevation of the projecting portion 340can have a desired value of, e.g., about 100 μm. Hence, the projectingportion 340 can have a large tilt angle so that the mirror 230 can havea pivot angle larger than before.

Especially, in the electrode substrate shown in FIG. 21, the differenceof elevation from the upper surface 320 a of the base 320 to the pivot350 can be smaller than in the electrode substrate shown in FIGS. 16Aand 16B. For this reason, in pattern transfer by the exposure apparatusin forming the pivot 350, the defocus in the vertical direction for theupper surface 320 a of the base 320 can be smaller. Consequently, moreaccurate pattern formation is possible.

The mirror array having an outer trench with a multistage structureshown in FIG. 21 can be manufactured by the same manufacturing method asthat for the above-described mirror array shown in FIGS. 14 to 16B.

The outer trench need not always have the above-described integralstructure but may have a divided structure. FIG. 22 shows a mirror arrayhaving an outer trench with a divided structure. FIG. 22 is a sectionalview showing a modification of the electrode substrate of the mirrorarray according to this embodiment. The same names and referencenumerals as in the mirror device shown in FIGS. 14 to 16B denote thesame constituent elements in FIG. 22, and a description thereof will beomitted as needed.

An electrode substrate 302 shown in FIG. 22 has the plate-shaped base320, four outer trenches 333 a to 333 d each including a concave portionformed on the base 320 and having an almost L-shape when viewed from theupper side, and the almost conical projecting portion 340 formed on thebase 320. An insulating film (not shown) is formed on the surface of thebase 320 with the outer trenches 333 a to 333 d and projecting portion340. The four sector electrodes 360 a to 360 d are formed on the uppersurfaces of the mirror 230 with the insulating film, the outer trenches333 a to 333 d, and the projecting portion 340 to form a circle that isconcentric to the mirror 230. The pair of convex portions 370 a and 370b are formed on the upper surface 320 a of the base 320 to sandwich theouter trenches 333 a to 333 d. The interconnections 380 formed on theupper surface 320 a of the base 320 between the convex portions 370 aand 370 b and the outer trenches 333 a to 333 d. The interconnections380 connect to the electrodes 360 a to 360 d through the leads 361 a to361 d formed on the base 320.

Each of the outer trenches 333 a to 333 d includes a concave portionformed in the base 320. The concave portion has an inverted truncatedpyramidal shape with an upper surface larger than the bottom surface.The outer trenches 333 a to 333 d are formed point-symmetrically tosurround the projecting portion 340 and form a rectangle concentric tothe projecting portion 340 and electrodes 360 a to 360 d.

The projecting portion 340 includes the second terrace 342 having atruncated pyramidal shape and formed on the upper surface 320 a of thebase 320 surrounded by the outer trenches 333 a to 333 d, the firstterrace 341 formed on the upper surface of the second terrace 342 andhaving a truncated pyramidal shape, and the pivot 350 formed on theupper surface of the first terrace 341 and having a truncated pyramidalshape. The lower surface of the second terrace 342 is flush with theupper surface of the base 320, as a matter of course. A portionsurrounded by the outer trenches 333 a to 333 d and having an almosttruncated pyramidal shape with an upper surface corresponding to theupper surface 320 a of the base 320 is equivalent to the third terrace343 shown in FIGS. 14 to 16B and 21.

The projecting portion 340 can be formed such that its difference ofelevation obtains a desired value. For example, when the projectingportion 340 includes the almost truncated pyramidal portioncorresponding to the third terrace, the projecting portion 340 can havea difference of elevation of, e.g., about 100 μm. Hence, the projectingportion 340 can have a large tilt angle so that the mirror 230 can havea pivot angle larger than before.

Each of the leads 361 a to 361 d is formed between adjacent two of theouter trenches 333 a to 333 d on the base 320. Hence, the leads 361 a to361 d and interconnections 380 are formed on the base 320.

Even in FIG. 22, in transferring a pattern serving as the electrodes 360a to 360 d, leads 361 a to 361 d, and interconnections 380 to aphotoresist, the exposure apparatus sets the focus on the upper surfaceof the base 320. Hence, the interconnections 380 can be formed at anaccuracy of several μm. Especially in FIG. 22, since the leads 361 a to361 d are formed on the base 320, not only the interconnections 330 butalso the leads 361 a to 361 d can be formed at an accuracy of severalμm. Even for the electrodes 360 a to 360 d, pattern transfer is done ina region with relatively small image defocus within or near the rangewhere the exposure apparatus can precisely transfer the pattern. It istherefore possible to accurately form the electrodes 360 a to 360 d.

As a result, the difference of elevation of the projecting portion 340can have a desired value. Hence, the projecting portion 340 can have alarge tilt angle so that the mirror 230 can have a pivot angle largerthan before.

The mirror array having divided outer trenches shown in FIG. 22 can bemanufactured by the same manufacturing method as that for theabove-described mirror array shown in FIGS. 14 to 16B.

In this embodiment, the electrode substrate 300, 301, or 302 having theprojecting portion 340 and the like is formed by etching the siliconsubstrate 400. Instead, the electrode substrate 300, 301, or 302 havingthe projecting portion 340 and the like may be formed by depositing anarbitrary substance on an arbitrary substrate.

As described above, according to this embodiment, since a projectingportion is formed in a trench formed in the base to increase thedifference of elevation of the projecting portion, the mirror can have alarge pivot angle. Interconnections are formed on the base, and theelectrodes are formed on the trench formed in the substrate and theprojecting portion projecting from the trench. Hence, setting the focusof the exposure apparatus on the upper surface of the base allows toform the interconnections and electrodes at a necessary accuracy.

Ninth Embodiment

A conventional mirror device will be described first. As shown in FIG.13, in a conventional mirror device 8200, an electric field generated byapplying individual voltages to electrodes 8340 a to 8340 d throughinterconnections 8370 gives an attracting force to a mirror 8230 andcauses it to pivot by an angle of several degrees. When no voltages areapplied to the electrodes 8340 a to 8340 d, the mirror 8230 is almostparallel to an electrode substrate 8301 (this state will be referred toas an initial position hereinafter), as indicated by the solid line inFIG. 13. When individual voltages are applied to the electrodes 8340 ato 8340 d in this state, the mirror 8230 tilts, as indicated by thedotted line in FIG. 13.

FIG. 23 shows an optical switch having the mirror device 8200. Anoptical switch 8400 includes a pair of collimator arrays 8410 and 8420each having a plurality of optical fibers arrayed two-dimensionally, anda pair of mirror arrays 8430 and 8440 each having a plurality ofabove-described mirror devices 8200 arrayed two-dimensionally. In theoptical switch 8400, a light beam input from the collimator array 8410serving as an input port is reflected by the mirror arrays 8430 and8440, reaches the collimator array 8420 serving as an output port, andexits it. For example, a light beam a that has entered from an opticalfiber 8410 a of the collimator array 8410 into the optical switch 8400irradiates a mirror device 8200 a of the mirror array 8430. The lightbeam is reflected by the mirror 8230 of the mirror device 8200 a andreaches a mirror device 8200 b of the mirror array 8440. In the mirrorarray 8440, the light beam is reflected by the mirror 8230 of the mirrordevice 8200 b and reaches an optical fiber 8420 a of the collimatorarray 8420, as in the mirror array 8430. In this way, the optical switch8400 can spatially cross-connect the collimated light input from thecollimator array 8410 to the collimator array 8420 in the form of alight beam without converting it into an electrical signal.

In the optical switch 8400, identical constituent elements of the mirrordevices 8200 two-dimensionally arranged in the mirror arrays 8430 and8440 have the same shape. For example, the size of the mirror 8230, theshapes of movable frame connectors 8211 a and 8211 b and mirrorconnectors 8221 a and 8221 h, the relative positions between the mirror8230 and the movable frame connectors 8211 a and 8211 b or between themirror 8230 and the mirror connectors 8221 a and 8221 b, the relativepositions between the mirror 8230 and the electrodes 8340 a to 8340 d,and the sizes of the electrodes 8340 a to 8340 d do not change betweenthe mirror devices. As described above, in the conventional opticalswitch 8400, the mirror devices 8200 having the same shape are arrangedin the mirror arrays 8430 and 8440. For this reason, each mirror 8230 inthe mirror arrays 8430 and 8440 is almost parallel to the electrodesubstrate 8301, as shown in FIG. 24. FIG. 24 schematically shows thesections of the mirrors 8230 of the plurality of mirror devices 8200arranged in the mirror arrays 8430 and 8440. The mirror array 8430 hasmirrors 8431 to 8435 corresponding to the mirror 8230. The mirror array8440 has mirrors 8441 to 8445 corresponding to the mirror 8230.

The mirrors 8431 to 8435 of the mirror array 8430 are almost parallel tothe electrode substrate 8301 at the initial position and are arrangedsuch that a light beam irradiates corresponding mirrors in the countermirror array 8440. For example, the mirror 8431 reflects a light beamindicated by a (to be referred to as a light beam a hereinafter) in FIG.24 to the mirror 8441 as the counterpart of the mirror 8431. The mirror8433 reflects a light beam indicated by b (to be referred to as a lightbeam b hereinafter) in FIG. 24 to the mirror 8443 as the counterpart ofthe mirror 8433. The mirror 8435 irradiates the mirror 8445 as thecounterpart of the mirror 8435 with a light beam indicated by c (to bereferred to as a light beam c hereinafter) in FIG. 24. A counter mirrorindicates, e.g., a mirror located at the same position in the countermirror ray when one mirror array is projected from the back of the beamreflection surface, and the other mirror ray is viewed from the beamreflection surface.

When individual voltages are applied to the electrodes 8340 a to 8340 dof the mirrors 8431 to 8435 in the mirror array 8430 of the opticalswitch 8400 to tilt the mirrors 8431 to 8435, the light beams thatirradiate the mirrors 8431 to 8435 can irradiate arbitrary mirrors inthe mirror array 8440.

In the conventional mirror array, however, if a light beam shouldirradiate a two-dimensional target by tilting the mirrors of the mirrordevices, the mirror tilt angle changes depending on the position of themirror device in the mirror array.

For example, in the optical switch 8400 having the pair of mirror arrays8430 and 8440 shown in FIG. 24, when the mirror 8431 tilts by ½ of θ1 inthe positive direction (to the right in FIG. 24 viewed from the frontside) from the initial, position, the light beam a irradiates the mirror8445 of the mirror array 8440 as a target. Hence, to irradiate all themirrors 8441 to 8445 of the counter mirror array 8440 with the lightbeam a, the mirror 8431 must be able to tilt by ½ of θ1 in the positivedirection from the initial position.

When the mirror 8433 tilts by ½ of θ2 in the negative direction (to theleft in FIG. 24 viewed from the front side) from the initial position,the light beam b irradiates the mirror 8441. When the mirror 8433 tiltsby ½ of θ3 in the positive direction from the initial position, thelight beam b irradiates the mirror 8445. Hence, to irradiate all themirrors 8441 to 8445 of the counter mirror array 8440 with the lightbeam b, the mirror 8433 must be able to tilt by ½ of θ2 in the negativedirection from the initial position and by ½ of θ3 in the positivedirection from the initial position.

When the mirror 8435 tilts by ½ of θ4 in the negative direction from theinitial position, the light beam c irradiates the mirror 8441. Hence, toirradiate all the mirrors 8441 to 8445 of the counter mirror array 8440with the light beam c, the mirror 8435 must be able to tilt by ½ of θ4in the negative direction from the initial position.

As described above, in the conventional mirror array, the mirror tiltangle changes depending on the position in the mirror array. However,since the mirror devices have the same shape, all the mirror devicesmust be able to tilt by the same angles necessary for the devicesincluded in the same mirror array. For example, the mirrors 8431 to 8435in FIG. 24 must be able to tilt by ½ of θ1 in the positive direction andby ½ of θ4 in the negative direction. However, since the mirror deviceusing the MEMS technology has difficulty in increasing the mirror tiltangle at a low voltage, a demand has arisen for a mirror device capableof decreasing the mirror tilt angle.

This embodiment has been made to solve the above-described problem andhas as its object to provide a mirror device capable of decreasing themirror tilt angle, a mirror array, and an optical switch.

This embodiment will be described next. In this embodiment, the positionof a fulcrum projection shifts from the center of a mirror to decreasethe mirror tilt angle. The same reference numerals as in the eighthembodiment denote the same constituent elements in the ninth embodiment.FIGS. 25A and 25B mainly partially illustrate a mirror device having amirror as a constituent unit of a mirror array.

A mirror device 2 according to this embodiment has a structure in whicha mirror substrate 200 with a mirror and an electrode substrate 300 withelectrodes are arranged in parallel. As shown in FIG. 25A, a pivot(fulcrum projection) 350 formed from an almost columnar projectionexists on the upper surface of a first terrace 341 of the electrodesubstrate 300 at a point shifted from the center of the upper surface. Aprojecting portion 320 of the electrode substrate 300 faces a mirror 230of the mirror substrate 200. Hence, the position of the pivot 350 shiftsfrom the central axis perpendicular to the plane of the mirror 230. Thatis, when the center of the mirror 230 is projected to the electrodesubstrate 300, the center of the mirror 230 and the pivot 350 arelocated at different points on the electrode substrate 300 withoutcoincidence. The distance and direction of movement of the pivot 350from almost the center of the first terrace 341 are set in accordancewith the position of the mirror device 2 in the mirror array.

As shown in FIG. 26, in the conventional mirror device 8200, a pivot8330 faces almost the center of the mirror 8230. For this reason, when auniform voltage is applied to the electrodes 8340 a to 8340 d, a uniformattracting force acts on the entire mirror 8230. The mirror 8230approaches the side of the electrode substrate 8301 and contacts thepivot 8330. The mirror 8230 is almost parallel to the electrodesubstrate 8301, i.e., perpendicular to the axis (indicated by thealternate long and short dashed line in FIG. 26) of the pivot 8330.

To the contrary, in the mirror device 2 of this embodiment, the positionof the pivot 350 shifts from the center of the mirror 230 facing thepivot 350. For this reason, when a uniform voltage (to be referred to asa bias voltage hereinafter) is applied to electrodes 360 a to 360 d, themirror 230 contacts the pivot 350 and tilts by a predetermined anglefrom a state (indicated by the dotted line in FIG. 27) wherein themirror perpendicular to the axis (indicated by the alternate long andshort dashed line in FIG. 27) of the pivot 350 (this state will bereferred to as an initial state hereinafter). In this embodiment,individual displacement voltages are applied to the electrodes 360 a to360 d in this state, thereby tilting the mirror 230 on the pivot 350.

A mirror array according to this embodiment and an optical switch havingthe mirror array will be described next.

As shown in FIG. 28, in a mirror array 500 according to this embodiment,mirror devices 2 described with reference to FIGS. 25A, 25B, and 27 aretwo-dimensionally arranged in a matrix.

As shown in FIG. 29, an optical switch 600 according to this embodimentincludes a pair of collimator arrays 610 and 620 each having a pluralityof optical fibers arrayed two-dimensionally, and a pair of mirror arrays510 and 520 each including the above-described mirror array 500. In theoptical switch 600, a light beam input from the collimator array 610serving as an input port is reflected by the mirror arrays 510 and 520,reaches the collimator array 620 serving as an output port, and exitsit. For example, light beam a that has entered from an optical fiber 610a of the collimator array 610 into the optical switch 600 irradiates amirror device 2-1 of the mirror array 510. The light beam is reflectedby the mirror 230 of the mirror device 2-1 and reaches a mirror device2-2 of the or array 520. In the mirror array 520, the light beam isreflected by the mirror 230 of the mirror device 2-2 and reaches anoptical fiber 620 a of the collimator array 620, as in the mirror array510. In this way, the optical switch 600 can spatially cross-connect thecollimated light input from the collimator array 610 to the collimatorarray 620 in the form of a light beam without converting it into anelectrical signal.

In the mirror array 500, the position (indicated by a dotted circle inFIG. 28) of the pivot 350 of each mirror device shifts from the center(indicated by x in FIG. 28) of the mirror 230 to reflect a light beam toa mirror located at the center of the counter mirror array in theinitial state. For example, in the mirror array 500 shown in FIG. 28,the pivot 350 of each mirror device 2 exists on a straight line thatconnects a mirror device 2 a located at the center of the mirror array500 to the center of the mirror 230 of the mirror device 2. As thedistance from the mirror device 2 a increases, the position of the pivot350 of each mirror device 2 gradually separates from the center of themirror 230 of the mirror device 2 to the opposite side of the mirrordevice 2 a. The pivot 350 of the mirror device 2 a exists at a positionfacing the center of the mirror 230. Hence, when a bias voltage isapplied to the electrodes 360 a to 360 d of each mirror device 2 of themirror array 500, the mirror 230 contacts the pivot 350 and tilts toreflect a received light beam to the mirror at the center of the countermirror array.

FIG. 30 is a schematic view of the sections of the mirror arrays 510 and520. FIG. 30 schematically illustrates the sections of the mirrors 230of the plurality of mirror devices 2 arranged in the mirror arrays 510and 520. Each of mirrors 511 to 515 of the mirror array 510 and mirrors521 to 525 of the mirror array 520 corresponds to the above-describedmirror 230.

When a bias voltage is applied to the electrodes 360 a to 360 d of eachmirror device 2 of the mirror array 510, each of the mirrors 511 to 515tilts at a predetermined angle depending on the position in the mirrorarray 510, as shown in FIG. 30. In this initial state, the mirrors 511to 515 tilt to reflect a tight beam input to the mirror array 510 to themirror 523 located at the center of the mirror array 520. To irradiatethe mirrors 521 to 525 of the mirror array 520 with a light beam, themirrors 511 to 515 need only be able to tilt by almost the same angle inthe positive direction (to the right in FIG. 30 viewed from the frontside) and negative direction (to the left in FIG. 30 viewed from thefront side).

For example, when the mirror 511 in FIG. 30 tilts by ½ of θ5 in thenegative direction from the initial state, the light beam a irradiatesthe mirror 521. When the mirror 511 tilts by ½ of θ6 in the positivedirection from the initial state, the light beam a irradiates the mirror525. Hence, to irradiate all the mirrors 521 to 525 of the countermirror array 520 with the light beam a, the mirror 511 need only be ableto tilt by ½ of θ5 in the negative direction and by ½ of θ6 in thepositive direction from the initial state.

When the mirror 515 tilts by ½ of θ7 in the negative direction from theinitial state, a light beam c irradiates the mirror 521. When the mirror515 tilts by ½ of θ8 in the positive direction from the initial state,the light beam c irradiates the mirror 525. Hence, to irradiate all themirrors 521 to 525 of the counter mirror array 520 with the light beamc, the mirror 515 need only be able to tilt by ½ of θ7 in the negativedirection and by ½ of θ8 in the positive direction from the initialstate.

In the conventional mirror array 8430 shown in FIG. 24, to irradiate allthe mirrors 8441 to 8445 of the counter mirror array 8440 with the lightbeam a, the mirror 8431 corresponding to the mirror 511 of the mirrorarray 510 of this embodiment must be able to tilt by ½ of θ1 (θ5+θ6) inthe positive direction from the initial position. Additionally, themirror 8435 corresponding to the mirror 515 of the mirror array 510 ofthis embodiment must be able to tilt by ½ of θ4 (=θ7+θ8) in the negativedirection from the initial position. Since the mirror devices 8200 ofthe conventional mirror array 8430 have the same shape, the mirrors 230must be able to tilt by ½ of θ1 in the positive direction and by ½ of θ4in the negative direction.

In this embodiment, however, for example, the mirror 511 need only beable to tilt by ½ of θ5 in the negative direction and by ½ of θ6 in thepositive direction from the initial state, as described above. Thisangle is smaller than and, more specifically, about ½ the tilt angle ofthe mirror 230 of the mirror device 8200 in the conventional mirrorarray 8430. As described above, the mirror array according to thisembodiment can decrease the mirror tilt angle. Hence, the drivingvoltage of the mirror device and mirror array can be low.

A method of manufacturing the mirror device and mirror array accordingto this embodiment will be described next. The mirror substrate 200 isformed from an SOI (Silicon On Insulator) substrate.

First, a side (major surface: SOI layer) of the SOI substrate with aburied insulating layer 250 undergoes known photolithography and etchingsuch as DEEP RIE to form, in the single-crystal silicon layer, trenchesconforming to the shapes of a frame portion 210, movable frameconnectors 211 a and 211 b, movable frame 220, or connectors 221 a and221 b, and mirror 230.

A resist pattern with openings in predetermined regions corresponding tothe trenches is formed on the lower surface of the SOI substrate. Thesilicon is selectively etched from the lower surface of the SOIsubstrate by dry etching using, e.g., SF₆. In this etching, an openingand a frame-shaped member 240 are formed on the lower surface of the SOIsubstrate by using the buried insulating layer 250 as an etching stopperlayer. The etching of silicon may be wet etching using, e.g., potassiumhydroxide.

A region of the buried insulating layer 250 exposed to the opening isremoved by dry etching using, e.g., CF₆. With this process, the mirrorsubstrate 201 is formed. The buried insulating layer 250 may be removedby using hydrofluoric acid.

On the other hand, the electrode substrate 8301 is formed from, e.g., asilicon substrate. First, a silicon substrate is selectively etched byusing, as a mask, a predetermined mask pattern made of a silicon nitridefilm or silicon oxide film and a potassium hydroxide solution. A base310, first and second terraces 321 and 322, pivot 350, and convexportions 360 a and 360 b are formed by repeating the above-describedprocess. The pivot 350 is formed at a position shifted from the centerof a first terrace 341 depending on the position in the mirror array500. The pivot 350 of the mirror device 2 at the center of the mirrorray 500 is formed almost at the center of the first terrace 341.

The surface of the silicon substrate on the etched side is oxidized toform a silicon oxide film. A metal film is formed on the silicon oxidefilm by, e.g., vapor deposition and patterned by known photolithographyand etching to form the electrodes 360 a to 360 d, leads 341 a to 341 d,and interconnections 370. With this process, the electrode substrate 300having the above-described shape is formed.

Then, the mirror substrate 20 is bonded to the electrode substrate 300to form the mirror array 500 having the mirror device 2 that moves themirror 230 by applying an electric field to the electrodes 360 a to 360d. The position of the pivot 350 of each mirror device 2 of the thusmanufactured mirror array 500 is adjusted in accordance with itsposition in the mirror array 500. Each mirror device reflects a lightbeam to the mirror at the center of the counter mirror array 520 uponapplying a bias voltage to the electrodes 360 a to 360 d. This allows todecrease the tilt angle of the mirror 230 of each mirror device 2.

In this embodiment, the mirror array 500 shown in FIG. 28 has 5×5 mirrordevices 2. However, the number of mirror devices 2 provided in themirror 500 is not limited to 5×5 and can freely be set as needed.

The mirror array 520 according to this embodiment may have the samestructure as the mirror array 510.

The mirror 230 of the mirror device 2 according to this embodiment neednot always tilt one-dimensionally, as shown in FIG. 30. The mirror 230may tilt two-dimensionally about the movable frame pivot axis and themirror pivot axis. Hence, the position of the pivot 350 of the mirrordevice 2 on the first terrace 341 is two-dimensionally adjusteddepending on the position in the mirror array 500 of the mirror device2.

In this embodiment, the pivot 350 is provided on the projecting portion320. However, the pivot 350 may exist on the electrode substrate 300. Inthis case, the electrodes 360 a to 360 d are formed on the electrodesubstrate 300.

In this embodiment, a bias voltage and displacement electrodes areapplied to the electrodes 360 a to 360 d. However, only the displacementvoltages may be applied.

In this embodiment, no third terrace 343 is formed on a projectingportion 340. However, the present invention is also applicable to amirror device with the third terrace 343. Similarly, although no outertrench is formed in this embodiment, the present invention is alsoapplicable to a mirror device with an outer trench.

The mirror device 2 and mirror array according to this embodiment areusable not only in an optical switch but also in a measurement device,display, and scanner. In this case, the pivot 350 of the mirror device 2is provided at an arbitrary position in accordance with the applicationpurpose and specifications.

As described above, according to this embodiment, since the position ofthe fulcrum projection shifts from the center of the mirror, the mirrortilts by a predetermined angle when a bias voltage is applied to theelectrodes. Since the mirror tilts from this tilted state, the tiltangle of the mirror can be small. This also allows to drive the mirrordevice at a low voltage.

10th Embodiment

The 10th embodiment of the present invention will be described next. Inthis embodiment, a connector and a counter connector have differentstructures to decrease the mirror tilt angle.

The same reference numerals as in the eighth and ninth embodimentsdenote the same constituent elements in the 10th embodiment. FIG. 31mainly partially illustrates a mirror device 2 having a mirror as aconstituent unit of a mirror array. A mirror array according to thisembodiment includes a plurality of mirror devices two-dimensionallyarranged in a matrix.

The mirror device 2 has a structure in which a mirror substrate 200 witha mirror and an electrode substrate 300 with electrodes are arranged inparallel. The mirror substrate 200 has a plate-shaped frame portion 210with an opening having an almost circular shape when viewed from theupper side, a movable frame 220 with an opening having an almostcircular shape when viewed from the upper side and arranged in theopening of the frame portion 210 by a pair of movable frame connectors211 a and 211 b, a mirror 230 having an almost circular shape whenviewed from the upper side and arranged in the opening of the movableframe 220 by a pair of mirror connectors 221 a and 221 b, and aframe-shaped member 240 formed on the upper surface of the frame portion210 to surround the movable frame 220 and mirror 230.

The pair of movable frame connectors 211 a and 211 b including zigzagtorsion springs and provided in first notches 222 a and 222 b of themovable frame 220 connect the frame portion 210 to the movable frame220. This structure makes the movable frame 220 pivotable about amovable frame pivot axis passing through the pair of movable frameconnectors 211 a and 211 b. The movable frame connectors 211 a and 211 bhave different structures and, more particularly, different springconstants. In, e.g., FIG. 31, the movable frame connector 211 a isthicker than the movable frame connector 211 b. Hence, the springconstant of the movable frame connector 211 a is larger than that of themovable frame connector 211 b.

The pair of mirror connectors 221 a and 221 b including zigzag torsionsprings and provided in second notches 223 a and 223 b of the movableframe 220 connect the movable frame 220 to the mirror 230. Thisstructure makes the mirror 230 pivotable about a mirror pivot axispassing through the pair of mirror connectors 221 a and 221 b. Themirror connectors 221 a and 221 b have different structures and, moreparticularly, different spring constants. In, e.g., FIG. 31, the mirrorconnector 221 a is thicker than the mirror connector 221 b. Hence, thespring constant of the mirror connector 221 a is larger than that of themirror connector 221 b. The movable frame pivot axis and mirror pivotaxis intersect each other at a right angle.

The order of the spring constants of the movable frame connectors 211 aand 211 b and mirror connectors 221 a and 221 b will be described withreference to FIG. 32. FIG. 32 is a schematic view of a torsion springincluded in the movable frame connectors 211 a and 211 b and mirrorconnectors 221 a and 221 b. The spring constants of the movable frameconnectors 211 a and 211 b and mirror connectors 221 a and 221 b of themirror array according to this embodiment mainly indicate springconstants in the distance direction of the mirror substrate 200 andelectrode substrate 300, i.e., the Z-axis direction. The order of thespring constants in the Z-axis direction depends on the followingelements.

The order of the spring constants in the Z-axis direction depends on anX-direction length 11 of the torsion spring in FIG. 32. The longer thelength 11 is, the smaller the spring constant is. Reversely, the shorterthe length 11 is, the larger the spring constant is.

The order of the spring constants in the Z-axis direction also dependson a Y-direction length 12 of the torsion spring in FIG. 32. The longerthe length 12 is, the smaller the spring constant is. Reversely, theshorter the length 12 is, the larger the spring constant is.

The order of the spring constants the Z-axis direction also depends on awidth t of the torsion spring in FIG. 32. The smaller the width t is,i.e., the thinner the torsion spring is, the smaller the spring constantis. Reversely, the larger the width t is, i.e., the thicker the torsionspring is, the larger the spring constant is.

The order of the spring constants in the Z-axis direction also dependson an interval p between adjacent torsion spring elements, i.e., thepitch of the zigzag of the torsion spring in FIG. 32. The larger theinterval p is, the smaller the spring constant is. Reversely, thesmaller the interval p is, the larger the spring constant is.

The order of the spring constants in the Z-axis direction also dependson a number n of times of turn of the zigzag of the torsion spring inFIG. 32. The larger the number n is, i.e., the larger the number oftimes of turn of the torsion spring is, the smaller the spring constantis. Reversely, the smaller the number n is, i.e., the smaller the numberof times of turn of the torsion spring is, the larger the springconstant is.

The order of the spring constants in the Z-axis direction also dependson a thickness h of the torsion spring in FIG. 32. The smaller thethickness h is, i.e., the thinner the torsion spring is, the smaller thespring constant is. Reversely, the larger the thickness h is, i.e., thethicker the torsion spring is, the larger the spring constant is.

In the mirror array according to this embodiment, at least one of theabove-described elements changes between a pair of members, i.e., themovable frame connectors 211 a and 211 b and minor connectors 221 a and221 b. This causes each of the movable frame connectors 211 a and 211 band mirror connectors 221 a and 221 b to have a spring constantdifferent from that of the counterpart.

The spring constants in the X and Y directions and the spring constantof rotation about the X-axis also change depending on the shape of thetorsion spring, like the order of the above-described spring constantsin the Z-axis direction. This will be described with reference to FIGS.33A to 34B. FIGS. 33A to 34B show the relationship between the width andthe spring constant of a torsion spring made of, e.g., Si and having asimple beam structure with a rectangular section. Referring to FIGS. 33Aand 34A, the ordinate on the right side represents the spring constantof rotation about the longitudinal axis, i.e., the X-axis of the torsionspring. The ordinate on the left side represents the spring constant inthe height direction, i.e., the Z-axis direction of the torsion spring.The abscissa represents the width, i.e., Y-direction length of thetorsion spring. Referring to FIGS. 33A and 34A, a curve a indicates therelationship between the spring constant of rotation about the X-axis ofthe torsion spring and the width of the torsion spring. A curve bindicates the relationship between the spring constant in the Z-axisdirection of the torsion spring and the width of the torsion spring.

As indicated by the curves a in FIGS. 33A and 34A, the smaller the widthof the torsion spring is, the smaller the spring constant of rotationabout the X-axis of the torsion spring is. The larger the width of thetorsion spring is, the larger the spring constant is. When a parametersuch as the torsion spring width related to the shape of the torsionspring changes, not only the above-described spring constant in theZ-axis direction of the torsion spring but also the spring constants inthe X and Y directions and the spring constant of rotation about theX-axis change. If the spring constant of rotation about the X-axischanges, a voltage necessary for tilting the mirror to a predeterminedangle also changes. When the spring constant in the Z-axis direction ofthe torsion spring should change, it is necessary to set a torsionspring shape that does not change the spring constants in the X and Ydirections and the spring constant of rotation about the X-axis of thetorsion spring. To do this for example, the spring constants in the Xand Y directions and the spring constant of rotation about the X-axis ofthe torsion spring are fixed to arbitrary values.

For example, in a torsion spring with an X-direction length of 180 μmand a height of 10 μm in FIG. 33B, the spring constant of rotation aboutthe X-axis is set to 1.0×10⁻⁸. As shown in FIG. 33A, the torsion springwidth is calculated as 1.8 μm from the curve a, and the spring constantin the Z-axis direction is calculated as 69 from the curve b. Assumethat the X-direction length of the torsion spring in FIG. 33B changes to210 μm, as shown in FIG. 34B, and the spring constant of rotation aboutthe X-axis is 1.0×10⁻⁸, like the torsion spring in FIG. 33B. As shown inFIG. 34A, the torsion spring width is calculated as 1.9 μm from thecurve a, and the spring constant in the Z-axis direction is calculatedas 47 from the curve b.

As described above, when a parameter such as the torsion spring width orlength related to the shape of the torsion spring changes, the springconstant in the Z-axis direction of the torsion spring can changewithout changing the spring constant of rotation about the X-axis of thetorsion spring.

The operation of the mirror device of the mirror array according to thisembodiment will be described next with reference to FIGS. 31, 35, and36. FIGS. 35 and 36 illustrate the sections of the main part taken alonga line I-I in FIG. 31 so as to explain the relationship between themirror 230 and the mirror connectors 221 a and 221 b. FIGS. 35 and 36explain a mirror device having flat electrodes without any projectingportion as an example. However, this embodiment is also applicable to amirror device having a projecting portion. FIGS. 35 and 36 also explaina mirror device without any outer trench as an example. However, thisembodiment is also applicable to a mirror device having an outer trench.

The one-dimensional tilting operation of the mirror device will bedescribed first. In a conventional mirror device 8200 shown in FIG. 35,mirror connectors 8221 a and 8221 b have the same structure. Hence, twoends of a mirror 8230 supported by the mirror connectors 8221 a and 8221b are connected to a movable frame 8220 by the same suspending force.For this reason, when a uniform voltage is applied to electrodes 8340 ato 8340 d, a uniform attracting force acts on the entire mirror 8230.The mirror 8230 becomes almost parallel to a base 8310, as indicated bythe dotted line in FIG. 35.

In the mirror device 2 of this embodiment shown in FIG. 36, the mirrorconnectors 221 a and 221 b serving as a pair of members have differentstructures and, more particularly, different spring constants. Hence,two ends of the mirror 230 supported by the mirror connectors 221 a and221 b are connected to the movable frame 220 by different suspendingforces. For this reason, when a uniform voltage (to be referred to as abias voltage hereinafter) is applied to the electrodes 340 a to 340 d tomake an attracting force act on the mirror 230, the mirror 230 tilts bya predetermined angle with respect to a base 310 (this state will bereferred to as an initial state hereinafter), as indicated by the dottedline in FIG. 36.

The two-dimensional tilting operation of the mirror device will bedescribed next. In the mirror device 2 shown in FIG. 31, the movableframe connector 211 a has a spring constant larger than that of themovable frame connector 211 b, and the mirror connector 221 a has aspring constant larger than that of the mirror connector 221 b. For thisreason, when a bias voltage is applied to the electrodes 340 a to 340 d,the movable frame 220 that is parallel to the frame portion 210 tilts tothe side of the base 300 while reducing the distance to the base 300 inthe Y direction (this motion will be referred to as “tilt in the Ydirection” hereinafter). The mirror 230 that is parallel to the movableframe 220 tilts to the side of the base 300 while reducing the distanceto the base 300 in the X direction (this motion will be referred to as“tilt in the X direction” hereinafter). Hence, in the mirror device 2 ofthis embodiment, when a voltage with a uniform magnitude is applied tothe electrodes 340 a to 340 d, the mirror 230 that is parallel to theframe portion 210 tilts to the side of the base 310 while reducing thedistance to the base 300 in a direction of an arrow a in FIG. 31 (thismotion will be referred to as “tilt in the a direction” hereinafter). Inthis embodiment, the mirror 230 is tilted by applying individual controlvoltages to the electrodes 340 a to 340 d in this state.

FIG. 37 shows an example of a mirror array including the mirror devices2 according to this embodiment. FIG. 37 illustrates a state wherein apredetermined bias voltage is applied to the electrodes 340 a to 340 d.Each mirror device shown in FIG. 37 is in the state shown in FIG. 31viewed from the front side. That is, each mirror device has the mirrorconnectors 221 a and 221 b in the horizontal direction and the movableframe connectors 211 a and 211 b in the vertical direction.

In a mirror array 700 according to this embodiment, the mirror devices 2described with reference to FIGS. 31 and 36 are two-dimensionallyarranged in a matrix. The mirror array 700 corresponds to the mirrorarrays 510 an 520 of the optical switch 600 shown in FIG. 29. In themirror array 700, each of the movable frame connectors 211 a and 211 band mirror connectors 221 a and 221 b in each mirror device has a springconstant different from the counterpart so that the mirror 230 reflectsa light beam to a mirror located at the center of the counter mirrorarray in the initial state wherein a uniform bias voltage is applied tothe electrodes 340 a to 340 d. In the mirror device located at thecenter of the mirror array 700, each of the movable frame connectors 211a and 211 b and mirror connectors 221 a and 221 b has the same springconstant as the counterpart.

For example, in a mirror device 2 b adjacent to a mirror device 2 a in adirection (to be referred to as a “−X direction” hereinafter) reverse tothe X direction, the spring constant in the Z-axis direction of themirror connector 221 a is larger than that of the mirror connector 221 bsuch that the mirror 230 tilts in the X direction more than in themirror device 2 a in the initial state. In a mirror device 2 c adjacentto the mirror device 2 b in the −X direction, the mirror 230 tilts inthe X direction more than the mirror 230 of the mirror device 2 b in theinitial state. For example, if the mirror connectors 221 b of the mirrordevices 2 c and 2 b have the same spring constant in the Z-axisdirection, the spring constant in the Z-axis direction of the orconnector 221 a of the mirror device 2 c is set larger than that of themirror connector 221 b of the mirror device 2 b.

En a mirror device 2 d adjacent to the mirror device 2 a in a direction(to be referred to as a “−Y direction” hereinafter) reverse to the Ydirection, the spring constant in the Z-axis direction of the movableframe connector 211 a is larger than that of the movable frame connector211 b such that the mirror 230 tilts in the Y direction more than in themirror device 2 a in the initial state. In a mirror device 2 e adjacentto the mirror device 2 d in the −Y direction, the mirror 230 tilts inthe Y direction more than the mirror 230 of the mirror device 2 d in theinitial state. For example, if the movable frame connectors 211 b of themirror devices 2 d and 2 e have the same spring constant in the Z-axisdirection, the spring constant in the Z-axis direction of the movableframe connector 211 a of the mirror device 2 e is set larger than thatof the movable frame connector 211 b of the mirror device 2 d.

In a mirror device 2 f adjacent to the mirror device 2 a in the −X and−Y directions, i.e., in a direction (to be referred to as a “−adirection” hereinafter) reverse to the direction of the arrow a in FIG.37, the spring constant in the Z-axis direction of the movable frameconnector 211 a is larger than that of the movable frame connector 211b, and the spring constant in the Z-axis direction of the mirrorconnector 221 a is larger than that of the mirror connector 221 b suchthat the mirror 230 tilts in the X and Y directions, i.e., in thedirection of the arrow a (to be referred to as an “a direction”hereinafter) in FIG. 37 more than in the mirror device 2 a in theinitial state. In a mirror device 2 g adjacent to the mirror device 2 fin the −a direction, the mirror 230 tilts in the a direction more thanthe mirror 230 of the mirror device 2 f in the initial state. Forexample, if the movable frame connectors 211 b and mirror connectors 221b of the mirror devices 2 f and 2 g have the same spring constants inthe Z-axis direction, the spring constants in the Z-axis direction ofthe movable frame connector 211 a and connector 221 a of the mirrordevice 2 f are set larger than those of the movable frame connector 211b and mirror connector 221 b of the mirror device 2 f.

In a mirror device 2 h adjacent to the mirror device 2 c in the −Ydirection, the mirror 230 tilts in the X direction similarly as in themirror devices 2 c and 2 g and in the Y direction similarly as in themirror devices 2 d and 2 f in the initial state. For example, if themovable frame connectors 211 b and mirror connectors 221 b of the mirrordevices 2 c, 2 d, 2 f, 2 g, and 2 h have the same spring constants inthe Z-axis direction, the movable frame connector 211 a of the mirrordevice 2 h has the same spring constant in the Z-axis direction as thatof the movable frame connectors 211 a of the mirror devices 2 d and 2 f.In addition, the mirror connector 221 a of the mirror device h has thesame spring constant in the Z-axis direction as that of the mirrorconnectors 221 a of the mirror devices 2 c and 2 g.

As described above, the movable frame connectors 211 a and 211 b andmirror connectors 221 a and 221 b have spring constants set depending onthe position in the mirror array 700. When a bias voltage having auniform magnitude is applied to the electrodes 340 a to 340 d, themirror 230 of each mirror device 2 of the mirror array 700 tilts toreflect a received light beam to the mirror at the center of the countermirror array. This reflecting operation is the same as that described inthe ninth embodiment with reference to FIG. 30.

FIG. 30 is a sectional view taken along a line II-II in FIG. 37. FIG. 30schematically illustrates the sections of the mirrors 230 of theplurality of mirror devices 2 arranged in mirror arrays 510 and 520 eachincluding the mirror array 700. Each of mirrors 511 to 515 of the mirrorarray 510 and mirrors 521 to 525 of the mirror array 520 corresponds tothe mirror 230 of the mirror device 2 included in the above-describedmirror array 700. The mirror arrays 510 and 520 shown in FIG. 30correspond to the mirror arrays 510 and 520, respectively, of theoptical switch 600 shown in FIG. 29.

As described above, in this embodiment, each of the movable frameconnectors 211 a and 211 b and mirror connectors 221 a and 221 b has aspring constant different from that of the counterpart. Since the mirrortilts by a predetermined angle in the initial state, the mirror tiltangle can be small consequently.

The mirror device and mirror array according to this embodiment aremanufactured by the same manufacturing method the above-described ninthembodiment.

In this embodiment, in the step of forming, the single-crystal siliconlayer, trenches conforming the shapes of the frame portion 210, movableframe connectors 211 a and 211 b, movable frame 220, mirror connectors221 a and 221 b, and mirror 230, the movable frame connectors 211 a and211 b and mirror connectors 221 a and 221 b are formed while changingthe spring constant depending on the position in the mirror array.

The spring constants of the movable frame connectors 211 a and 211 b andmirror connectors 221 a and 221 b of each mirror device 2 of the thusmanufactured or array 700 are adjusted in accordance with its positionin the mirror array 700. Each mirror device reflects a light beam to themirror at the center of a counter mirror array 80 upon applying apredetermined bias voltage to the electrodes 340 a to 340 d. This allowsto decrease the tilt angle of the mirror 230 of each mirror device 2.

FIG. 38 shows a modification of the mirror substrate 200. In thisembodiment, as described above, the movable frame connectors 211 a and211 b are provided in the first notches 222 a, and 222 b formed in themovable frame 220. However, the movable frame connectors 211 a and 211 bneed not always exist there and may exist in third notches 224 a and 224b formed in the frame portion 210, as shown in FIG. 38.

The electrodes 340 a to 340 d need not always exist on the base 310 andmay exist on, e.g., projecting portion provided on the base 310.Alternatively, the electrodes 340 a to 340 d may exist the projectingportion and base 310.

In this embodiment, the mirror array 700 shown in FIG. 37 has 5×5 mirrordevices 2. However, the number of mirror devices 2 provided in themirror array 700 is not limited to 5×5 and can freely be set as needed.The mirror array 80 of this embodiment may have the same structure asthe mirror array 700.

The mirror device 2 and mirror array according to this embodiment areusable not only in an optical switch but also in a measurement device,display, and scanner. In this case, the projecting portion 320 andelectrodes 340 a to 340 d are provided at arbitrary positions inaccordance with the application purpose and specifications.

In this embodiment, the mirror 230 is tilted in the initial state byadjusting the spring constant in the Z-axis direction of a torsionspring included in each of the movable frame connectors 211 a and 211 band mirror connectors 221 a and 221 b. Instead, the spring constant ofrotation about the X-axis of the torsion spring may be adjusted. Eventhis can make the mirror 230 tilt in the initial state.

In this embodiment, a bias voltage and displacement electrodes areapplied to the electrodes 340 a to 340 d. However, only the displacementvoltages may be applied.

As described above, according to this embodiment, each connector has astructure different from that of the counter connector so that themirror tilts by a predetermined angle in accordance with bias voltageapplication to the electrodes. Since the mirror is tilted by applyingcontrol voltages in this tilted state, the mirror tilt angle can besmall. Even when it is necessary to tilt the mirror largely, it canoperate at a low voltage.

11th Embodiment

The 11th embodiment of the present invention will be described next.This embodiment reduces the mirror tilt angle by providing electrodes atarbitrary positions on a substrate asymmetrically with respect to thepivot axis of a mirror projected onto an electrode substrate. The samereference numerals as in the eighth to 10th embodiments denote the sameconstituent elements in the 11th embodiment. FIGS. 19A and 39R mainlypartially illustrate a mirror device 2 having a mirror as a constituentunit of a mirror array. In this embodiment, a mirror device having flatelectrodes without any projecting portion will be described as anexample.

The mirror device 2 has a structure in which a mirror substrate 200 witha circular mirror 230 and an electrode substrate 300 with electrodes 340a to 340 d are arranged in parallel. As shown in FIGS. 39A and 39B, theelectrodes 340 a to 340 d are formed at arbitrary positions on theelectrode substrate 300 asymmetrically with respect to at least one ofthe movable frame pivot axis and mirror pivot axis of the mirror 230projected onto the electrode substrate 300. The electrodes 340 a to 340d have a sector shape obtained by dividing a circle with the same sizeas the mirror 230 into four parts of uniform size along parting linesparallel to the movable frame pivot axis and mirror pivot axis. Thecenter of the electrodes 340 a to 340 d indicates the center of thecircle. The center passes through the intersection of the parting lines.The distance and direction of movement of the electrodes 340 a to 340 dfrom almost the center of the electrode substrate 300 are set inaccordance with the position of the mirror device 2 in the mirror array.The parting lines need not always include only straight lines but mayinclude arbitrary curves.

As shown in FIG. 40, in a conventional mirror device 8200, electrodes8340 a to 8340 d are formed at positions on a base 8310 symmetricallywith respect to the movable frame pivot axis and mirror pivot axis of amirror 8230 projected onto an electrode substrate 8301. Hence, as shownin FIG. 40, the central axis (indicated by the alternate long and shortdashed line in FIG. 40) of the mirror 8230 matches the central axis(indicated by the alternate long and two short dashed line in FIG. 40)of the electrodes 8340 a to 8340 d. For this reason, when a uniformvoltage is applied to the electrodes 8340 a to 8340 d, a uniformattracting force acts on the entire mirror 8230. The mirror 230 isalmost parallel to the major surface of the base 8310 of the electrodesubstrate 8301, i.e., almost perpendicular to the central axis of themirror 8230. The central axis of the mirror 230 indicates a straightline that passes through the center of the mirror 8230 and isperpendicular to the plane of the mirror 8230, i.e., parallel to thedistance direction between a mirror substrate 8201 and the electrodesubstrate 8301. The central axis of the electrodes 8340 a to 8340 dindicates a straight line that passes through the center of theelectrodes 8340 a to 8340 d and is parallel to the planes of theelectrodes 8340 a to 8340 d, i.e., the distance direction between amirror substrate 8201 and the electrode substrate 8301.

To the contrary, in the mirror device 2 of this embodiment, theelectrodes 340 a to 340 d are formed at positions on the base 310asymmetrically with respect to at least one of the movable frame pivotaxis and mirror pivot axis of the mirror 230 projected onto theelectrode substrate 300. Hence, as shown in FIG. 41, the central axis(indicated by the alternate long and short dashed line in FIG. 41) ofthe mirror 230 does not match the central axis (indicated by thealternate long and two short dashed line in FIG. 41) of the electrodes340 a to 340 d. For this reason, when a uniform voltage (to be referredto as a bias voltage hereinafter) is applied to the electrodes 340 a to340 d, an attraction acts on a portion facing the electrodes 340 a to340 d. The mirror 230 tilts by a predetermined angle (this state will bereferred to as an “initial state” hereinafter) from a state (indicatedby the dotted line in FIG. 41) wherein the mirror is perpendicular tothe central axis. In this embodiment, the mirror 230 is tilted byapplying individual control voltages to the electrodes 340 a to 340 d inthis state.

A mirror array according to this embodiment will be described next withreference to FIG. 42. In a mirror array 800 according to thisembodiment, the mirror devices 2 described with reference to FIGS. 39A,39B, and 41 are two-dimensionally arranged in a matrix. The mirror array800 corresponds to the mirror arrays 510 an 520 of the optical switch600 shown in FIG. 29. In the mirror array 800, the electrodes 340 a to340 d (indicated by the dotted line in FIG. 42) of each mirror deviceare formed at positions on the electrode substrate 300 asymmetricallywith respect to the pivot axis of the mirror 230 projected onto theelectrode substrate 300 so that the mirror 230 reflects a light beam toa mirror located at the center of the counter mirror array in theinitial state. It is also possible to adjust the tilt of the mirror 230by the magnitude of a bias voltage.

For example, in the mirror array 800 shown in FIG. 42, the electrodes340 a to 340 d of each mirror device 2 exist on a straight line thatconnects a mirror device 2 a located at the center of the mirror array800 to the center of the mirror 230 of the mirror device 2. As thedistance from the mirror device 2 a increases, the positions of theelectrodes 340 a to 340 d of each mirror device 2 gradually separatefrom the center of the mirror 230 of the mirror device 2 to the oppositeside of the mirror device 2 a. This arrangement increases the tilt ofthe mirror 230 as the distance between the mirror device 2 a and themirror device 2 increases. The central axis of the electrodes 340 a to340 d of the mirror device 2 a matches the central axis of the mirror230. When a bids voltage having a uniform magnitude is applied to theelectrodes 340 a to 340 d of each mirror device 2 of the mirror array800, the mirror 230 tilts to reflect a received light beam to the mirrorat the center of the counter mirror a ray. This reflecting operation isthe same as that described in the ninth embodiment with reference toFIG. 30.

FIG. 30 is a sectional view taken along a line I-I in FIG. 42. FIG. 30schematically illustrates the sections of the mirrors 230 of theplurality of mirror devices 2 arranged in mirror arrays 510 and 520 eachincluding the mirror array 800. Each of mirrors 511 to 515 of the mirrorarray 510 and mirrors 521 to 525 of the mirror array 520 corresponds tothe above-described mirror 230. The mirror arrays 510 and 520 shown inFIG. 30 correspond to the mirror arrays 510 and 520 of the opticalswitch 600 shown in FIG. 29, respectively.

As described above, in this embodiment, it is possible to reduce themirror tilt angle. This allows to lower the driving voltage of themirror device and mirror array.

The mirror device and mirror array according to this embodiment aremanufactured by the same manufacturing method as in the above-describedninth embodiment.

In this embodiment, in the step of patterning a metal film by knownphotolithography and etching to form the electrodes 340 a to 340 d,leads 341 a to 341 d, and interconnections 370, the electrodes 340 a to340 d are formed at positions shifted from the center of a base 310depending on the position in the mirror array 800. The electrodes 340 ato 340 d of the mirror device 2 located at the center of the mirrorarray 800 are formed almost at the center of the base 310. With thisprocess, the electrode substrate 300 having the above-described shape isformed.

The positions of the electrodes 340 a to 340 d of each mirror device 2of the thus manufactured mirror array 800 are adjusted in accordancewith its position in the mirror array 800. Each mirror device reflects alight beam to the mirror at the center of the counter mirror array 800upon applying a uniform bias voltage to the electrodes 340 a to 340 d.This allows to decrease the tilt angle of the mirror 230 of each mirrordevice 2. Additionally, since the mirror 230 tilts in the initial state,the operation range of the mirror 230 is small, and consequently, lowvoltage driving is possible.

As described above, according to this embodiment, the electrodes areprovided at arbitrary positions on the substrate asymmetrically withrespect to the pivot axis of the mirror projected onto the electrodesubstrate. The mirror tilts by a predetermined angle in accordance withbias voltage application to the electrodes. Since the mirror is tiltedby applying a control voltage in the tilted state, the mirror tilt anglecan be small. Since the tilt angle is small, low voltage driving ispossible.

12th Embodiment

The 12th embodiment of the present invention will be described next. Inthis embodiment, a projecting portion is provided on the electrodesubstrate 300 of the 11th embodiment, and electrodes 340 a to 340 d areprovided on the projecting portion and electrode substrate 300, as shownin FIGS. 43 and 44. Hence, the same names and reference numerals as inthe 11th embodiment denote the same constituent elements in the 12thembodiment, and a description thereof will be omitted as needed.

A mirror device 2 has a structure in which a mirror substrate 200 with amirror and an electrode substrate 300 with electrodes are arranged inparallel.

The electrode substrate 300 has a plate-shaped base 310 and a conicalprojecting portion 320 projecting from the surface (upper surface) ofthe base 310. The projecting portion 320 includes a second terrace 322having a truncated pyramidal shape and formed on the upper surface ofthe base 310, a first terrace 321 having a truncated pyramidal shape andformed on the upper surface of the second terrace 322, and a pivot 330having a columnar shape and formed on the upper surface of the firstterrace 321. The projecting portion 320 has the center formed at aposition shifted from the center of the base 310 depending on theposition in a mirror array. The distance and direction of movement ofthe projecting portion 320 from the center of the base 310 are et inaccordance with the position of the mirror device 2 in the mirror array.

The four sector electrodes 340 a to 340 d are formed on the uppersurface of the base 310 including the outer surface of the projectingportion 320 in a circle with the same size as a mirror 230 of thecounter mirror substrate 200. In the device 2 viewed from the upperside, the center of the electrodes 340 a to 340 d almost matches thecenter of the pivot 330. As described above, the position of theprojecting portion 320 shifts from the center of the base 310 dependingon the position in a mirror array 800. Since the central axes do notmatch, the electrodes 340 a to 340 d and the mirror 230 partiallyoverlap each other when viewed from the upper side.

In the mirror device 2 of this embodiment, the central axis (indicatedby the alternate long and short dashed line in FIG. 44) of the mirror230 shifts from the central axis (indicated by the alternate long andtwo short dashed line in FIG. 44) of the projecting portion 320 havingthe electrodes 340 a to 340 d on the surface, as shown in FIG. 44. Forthis reason, when a uniform bias voltage is applied to the electrodes340 a to 340 d, the mirror 230 tilts by a predetermined angle (thisstate will be referred to as an “initial state” hereinafter) from astate (indicated by dotted line in FIG. 44) wherein the mirror isperpendicular to the central axis. In this embodiment, the mirror 230 istilted by applying individual control voltages to the electrodes 340 ato 340 d in this state.

A mirror array 800 shown in FIG. 42 is formed by arranging the mirrordevices 2 two-dimensionally in a matrix. In the mirror array 800 havingthe mirror device 2 shown in FIG. 42, the central axis of the projectingportion 320 and the central axis of the electrodes 340 a to 340 d ofeach mirror device 2 exist on a straight line that connects a mirrordevice 2 a located at the center of the mirror array 800 to the centerof the mirror 230 of the mirror device 2. As the distance from themirror device 2 a increases, the positions of the central axes graduallyseparate from the center of the mirror 230 of the mirror device 2 to theopposite side of the mirror device 2 a. The central axis of theprojecting portion 320 and the central axis of the electrodes 340 a to340 d of the mirror device 2 a match the central axis of the mirror 230.When a bias voltage is uniformly applied to the electrodes 340 a to 340d of each mirror device 2 of the mirror array 800, the mirror 230 tiltsto reflect a received light beam to the mirror at the center of thecounter mirror array. This allows the mirror array according to thisembodiment to reduce the mirror tilt angle.

A method of manufacturing the mirror array according to this embodimentwill be described next. A method of manufacturing the mirror substrate200 is the same as in the 11th embodiment.

In this embodiment, in the step of forming the base 310, first andsecond terraces 321 and 322, pivot 330, and convex portions 360 a and360 b, the projecting portion 320 is formed at a position shifted fromthe center of the base 310 depending on the position in the mirrorarray. The projecting portion 320 of the mirror device 2 located at thecenter of the mirror array 800 is formed almost at the center of thebase 310. With this process, the electrode substrate 300 having theabove-described shape is formed.

The position of the projecting portion 320, i.e., the positions of theelectrodes 340 a to 340 d of each mirror device 2 of the thusmanufactured mirror array 800 are adjusted in accordance with itsposition in the mirror array 800. Each mirror device reflects a lightbeam to the mirror at the center of the counter mirror array 800 uponapplying a uniform bias voltage to the electrodes 340 a to 340 d. Thisallows to decrease the tilt angle of the mirror 230 of each mirrordevice 2.

In the 11th and 12th embodiments, the mirror array 800 shown in FIG. 42has 5×5 mirror devices 2. However, the number of mirror devices 2provided in the mirror array 800 is not limited to 5×5 and can freely beset as needed.

The mirror 230 of the mirror device 2 according to the 11th and 12thembodiments tilts not only one-dimensionally as shown in FIG. 30 butalso two-dimensionally about the movable frame pivot axis and mirrorpivot axis. Hence, the positions of the electrodes 340 a to 340 d andprojecting portion 320 of the mirror device 2 on the electrode substrate300 are two-dimensionally adjusted in accordance with the position ofthe mirror device 2 in the mirror array 800.

The mirror device 2 and mix array according to the 11th and 12thembodiments are usable not only in an optical switch but also in ameasurement device, display, and scanner. In this case, the projectingportion 320 and electrodes 340 a to 340 d of the mirror device 2 areprovided at arbitrary positions in accordance with the applicationpurpose and specifications.

In the 11th and 12th embodiments, the central axis of the mirror 230 isshifted from the central axes of the projecting portion 320 andelectrodes 340 a to 340 d by adjusting the position of the projectingportion 320 or the positions of the electrodes 340 a to 340 d on theelectrode substrate 300. However, the central axis of the mirror 230 maybe shifted from the central axes of the projecting portion 320 andelectrodes 340 a to 340 d by adjusting the position of the mirror 230 onthe mirror substrate 200.

In the 11th and 12th embodiments, a bias voltage and displacementelectrodes are applied to the electrodes 340 a to 340 d. However, onlythe displacement voltages may be applied.

As described above, according to this embodiment, the electrodes existat arbitrary positions on the substrate asymmetrically with respect tothe pivot axis of the mirror projected onto the electrode substrate. Themirror tilts by a predetermined angle in accordance with bias voltageapplication to the electrodes. Since the mirror is tilted by applyingcontrol voltages in this tilted state, the mirror tilt angle can besmall. Since the tilt angle is small, low voltage driving is possible.

13th Embodiment

The 13th embodiment of the present invention will be described next.This embodiment reduces the mirror tilt angle by inhibiting at least oneof the movable frame pivot axis and mirror pivot axis of a mirror devicefrom passing through the gravity center of a mirror. The same referencenumerals as in the eighth to 12th embodiments denote the sameconstituent elements in the 13th embodiment. FIG. 45 mainly partiallyillustrates a mirror device 2 having a mirror as a constituent unit of amirror array. In this embodiment, a mirror device having flat electrodeswithout any projecting portion will be described as an example.

The mirror device 2 has a structure in which a mirror substrate 200 witha mirror and an electrode substrate 300 with electrodes are arranged inparallel. The mirror substrate 200 has a plate-shaped frame portion 210with an opening having an almost circular shape when viewed from theupper side, a movable frame 220 with an opening having an almostcircular shape when viewed from the upper side and arranged in theopening of the frame portion 210 by a pair of movable frame connectors211 a and 211 b, a mirror 230 having an almost circular shape whenviewed from the upper side and arranged in the opening of the movableframe 220 by a pair of mirror connectors 221 a and 221 b, and aframe-shaped member 240 formed on the upper surface of the frame portion210 so as to surround the movable frame 220 and mirror 230.

The pair of movable frame connectors 211 a and 211 b including zigzagtorsion springs and provided in first notches 222 a and 222 b of themovable frame 220 connect the frame portion 210 to the movable frame220. This structure makes the movable frame 220 pivotable about amovable frame pivot axis passing through the pair of movable frameconnectors 211 a and 211 b in the mirror device 2 shown in FIG. 45, themovable frame connectors 211 a and 211 b connect to the frame portion210 and movable frame 220 such that the movable frame pivot axisindicated by an alternate long and short dashed line k in FIG. 45 passesthrough a gravity center G of the mirror 230. In this embodiment, sincethe mirror 230 has an almost circular shape when viewed from the upperside, the gravity center of the mirror 230 corresponds to the center ofthe circle of the outside shape of the mirror 230.

The pair of mirror connectors 221 a and 221 b including zigzag torsionsprings and provided in second notches 223 a and 223 b of the movableframe 220 connect the movable frame 220 to the mirror 230. Thisstructure makes the mirror 230 pivotable about a mirror pivot axispassing through the pair of mirror connectors 221 a and 221 b. In themirror device 2 shown in FIG. 45, the mirror connectors 221 a and 221 bare provided at positions translated from the mirror pivot axis of aconventional mirror device, which is indicated by a dotted line l andpasses through the gravity center G, to the side of the movable frameconnector 211 b, i.e., in the Y direction shown in FIG. 45. Since themirror pivot axis is translated in the Y direction from the conventionalmirror pivot axis l, the mirror pivot axis does not pass through thegravity center of the mirror 230, as indicated by an alternate long andtwo short dashed line m in FIG. 45. The movable frame pivot axis andmirror pivot axis intersect at a right angle.

The operation of the mirror device according to this embodiment will bedescribed next with reference to FIGS. 45 to 47. FIGS. 46 and 47correspond the section of a main part taken along a line I-I in FIG. 45and illustrate the relationship between the mirror pivot axis and themirror 230.

In a conventional mirror device 8200 shown in FIG. 46, mirror connectors8221 a and 221 b are arranged to make the pivot axis pass through thegravity center G of a mirror 8230. Hence, the mirror 8230 is supportedsymmetrically with respect to the mirror pivot axis. For this reason,when a uniform voltage is applied to electrodes 8340 a to 8340 d, auniform attracting force acts on the entire mirror 8230. The mirror 8230and a movable frame 8220 approach the side of a base 8310 while keepingthe almost parallel state to the base 8310, as indicated by the dottedline in FIG. 46.

To the contrary, in the mirror device 2 of this embodiment shown in FIG.47, since the mirror connectors 221 a and 221 b are provided at thepositions translated to the side of the movable frame connector 211 b,i.e., in the Y direction, as described above, the mirror pivot axis mdoes not pass through the gravity center G of the mirror 230. As shownin FIGS. 45 and 41, on the mirror pivot axis m, the distance between themirror pivot axis and the end of the mirror 230 on the side of themovable frame connector 211 a is longer than the distance between themirror pivot axis and the end of the mirror 230 on the side of themovable frame connector 211 b. When an attracting force acts on themirror 230 upon application of a uniform voltage (to be referred to as abias voltage hereinafter), the mirror 230 approaches to the side of themovable frame 220 and base 310 and tilts about the mirror pivot axis, asindicated by the dotted line in FIG. 47, so that the end on the side ofthe movable frame connector 211 a is closer to the side of the base 310(this state will be referred to as an initial state hereinafter). Inthis embodiment, individual control voltages are applied to theelectrodes 340 a to 340 d in this state, i.e., in the state wherein theend of the mirror 230 in a direction (to be referred to as a “−Ydirection” hereinafter) reverse to the Y direction in FIG. 45 approachesto the side of the base 310 (this state will be referred to as “tilt inthe −Y direction” hereinafter), thereby tilting the mirror 230.

The distance and direction of movement of the mirror connectors 221 aand 221 b in the mirror device 2 shown in FIGS. 45 and 47 are freely setas needed in accordance with the position of the mirror device 2 in themirror array.

In the mirror device 2 shown in FIG. 45, the mirror connectors 221 a and221 b simultaneously move in the Y direction to prevent the mirror pivotaxis from passing through the gravity center G of the mirror 230.Instead, one of the mirror connectors 221 a and 221 b may move in the Ydirection to prevent the mirror pivot axis from passing through thegravity center G of the mirror 230. This case will be described withreference to FIG. 48. FIG. 48 mainly partially illustrates the mirrorsubstrate of the mirror device 2 having a mirror as a constituent unitof a mirror array. The same names and reference numerals as in themirror device shown in FIG. 45 denote the same constituent elements inFIG. 48, and a description thereof will be omitted as needed.

In the mirror device 2 shown in FIG. 48, the mirror connector 221 a isprovided at a position shifted from the mirror pivot axis of the mirrordevice 8200, which is indicated by the dotted line l in FIG. 48 andpasses through the gravity center G, to the side of the movable frameconnector 211 b, i.e., in the Y direction. The end of the conventionalmirror pivot axis l on the side of the mirror connector 221 a moves inthe Y direction. That is, the mirror pivot axis indicated by thealternate long and two short dashed line m in FIG. 48 does not passthrough the gravity center of the mirror 230. The mirror pivot axis mand movable frame pivot axis k do not intersect at a right angle. Themirror pivot axis m intersects the movable frame pivot axis k at anarbitrary angle. On the movable frame pivot axis k, the distance betweenthe mirror pivot axis and the end of the mirror 230 on the side of themovable frame connector 211 a is longer than the distance between themirror pivot axis and the end of the mirror 230 on the side of themovable frame connector 211 b. When an attracting force acts on themirror 230 in accordance with application of a bias voltage with auniform magnitude to the electrodes 340 a to 340 d, the mirror 230 tiltsabout the mirror pivot axis in that intersects the movable frame pivotaxis k at an arbitrary angle so that the end on the side of the movableframe connector 211 a approaches to the side of the base 310. In thisembodiment, individual control voltages are applied to the electrodes340 a to 340 d in this initial state, i.e., in the state wherein the endof the mirror 230 in a direction (to be referred to as a “−a direction”hereinafter) reverse to the a direction perpendicular to the mirrorpivot axis m in FIG. 48 approaches to the side of the base 310 (thisstate will be referred to as “tilt in the −a direction” hereinafter),thereby tilting the mirror 230.

In the mirror device 2 shown in FIG. 48, the mirror connector 221 amoves in the Y direction. However, the mirror connector 221 b may movein the Y direction. The moving distance of the mirror connectors 221 aand 221 b is freely set as needed in accordance with the position of themirror device 2 in the mirror array. The moving direction of the mirrorconnectors 221 a and 221 b, i.e., the positive/negative sign of the Ydirection from the mirror pivot axis of the conventional mirror device8200 is freely set as needed in accordance with the position of themirror device 2 in the mirror array.

Each of the mirror connectors 221 a and 221 b may move in the Ydirection. In this case, the mirror connectors 221 a and 221 b canfreely move as needed as long as the mirror pivot axis does not passthrough the gravity center of the mirror 230.

A mirror device whose movable frame pivot axis does not pass through thegravity center of the mirror will be described next with reference toFIG. 49. FIG. 49 mainly partially illustrates the mirror substrate 200of the mirror device 2 having a mirror as a constituent unit of a mirrorarray. The same names and reference numerals as in the mirror deviceshown in FIG. 45 denote the same constituent elements in FIG. 49, and adescription thereof will be omitted as needed.

In the mirror device 2 shown in FIG. 49, the movable frame connectors211 a and 211 b are provided at positions shifted from the movable framepivot axis of the conventional mirror device 8200, which is indicated bya dotted line n in FIG. 49 and passes through the gravity center G, tothe side of the mirror connector 221 b, i.e., in the X direction. Themovable frame pivot axis that is translated for the conventional movableframe pivot axis n in the X direction does not pass through the gravitycenter of the mirror 230, as indicated by the alternate long and shortdashed line k in FIG. 49. On the movable frame pivot axis k, thedistance between the movable frame pivot axis and the end of the mirror230 on the side of the mirror connector 221 a is longer than thedistance between the mirror pivot axis and the end of the mirror 230 onthe side of the mirror connector 221 b. When an attracting force acts onthe mirror 230 upon application of a predetermined bias voltage to theelectrodes 340 a to 340 d, the mirror 230 tilts about the movable framepivot axis so that the end on the side of the mirror connector 221 aapproaches to the side of the base 310. In this embodiment, individualcontrol voltages are applied to the electrodes 340 a to 340 d in thisinitial state, i.e., in the state wherein the end in a direction (to bereferred to as a “−X direction” hereinafter) reverse to the X directionin FIG. 49 approaches to the side of the base 310 (this state will bereferred to as “tilt in the −X direction” hereinafter), thereby tiltingthe mirror 230.

In the mirror device 2 shown in FIG. 49, the moving distance of themovable frame connectors 211 a and 211 b is freely set as needed inaccordance with the position of the mirror device 2 in the mirror array.The moving direction of the movable frame connectors 211 a and 211 b,i.e., the positive/negative sign of the X direction from the movableframe pivot axis of the conventional mirror device 8200 is freely set asneeded in accordance with the position of the mirror device 2 in themirror array.

As in FIG. 48, in the mirror device 2 shown in FIG. 49, one of themovable frame connectors 211 a and 211 b may move to prevent the movableframe pivot axis from passing through the gravity center of the mirror230. In this case, when an attracting force acts on the mirror 230 uponapplication of a uniform bias voltage to the electrodes 340 a to 340 d,the mirror 230 tilts about the movable frame pivot axis that intersectsthe mirror pivot axis at an arbitrary angle so that the end on the sideof one of the mirror connectors 221 a and 221 b approaches to the sideof the base 310. Even in this case, the distance and direction ofmovement of the movable frame connectors 211 a and 211 b are freely setas needed in accordance with the position of the mirror device 2 in themirror array.

Each of the movable frame connectors 211 a and 211 b may move in the Xdirection. In this case, the movable frame connectors 211 a and 211 bcan freely move as needed as tong as the movable frame pivot axis doesnot pass through the gravity center of the mirror 230.

A mirror device whose movable frame pivot axis and mirror pivot axis donot pass through the gravity center G of the mirror will be describednext with reference to FIG. 50. FIG. 50 mainly partially illustrates themirror substrate 200 of the mirror device 2 having a mirror as aconstituent unit of a mirror array. The same names and referencenumerals as in the mirror devices shown in FIGS. 45 and 49 denote thesame constituent elements in FIG. 50, and a description thereof will beomitted as needed.

In the mirror device 2 shown in FIG. 50, the movable frame connectors211 a and 211 b are provided at positions shifted from the movable framepivot axis of the conventional mirror device 8200, which is indicated bythe dotted line n in FIG. 50 and passes through the gravity center G, tothe side of the mirror connector 221 b, i.e., in the X direction. Themirror connectors 221 a and 221 b are provided at positions shifted fromthe mirror pivot axis of the conventional mirror device 8200, which isindicated by the dotted line l in FIG. 50 and passes through the gravitycenter G, to the side of the movable frame connector 211 b, i.e., in theY direction. The movable frame pivot axis indicated by the alternatelong and short dashed line k and the mirror pivot axis indicated by thealternate long and two short dashed line m do not pass through thegravity center of the mirror 230 because they are translated for theconventional movable frame pivot axis n and mirror pivot axis l in the Xand Y directions, respectively. On the movable frame pivot axis k, thedistance between the movable frame pivot axis and the end of the mirror230 on the side of the mirror connector 221 a is longer than thedistance between the mirror pivot axis and the end of the mirror 230 onthe side of the mirror connector 221 b. On the mirror pivot axis m, thedistance between the mirror pivot axis and the end of the mirror 230 onthe side of the movable frame connector 211 a is longer than thedistance between the mirror pivot axis and the end of the mirror 230 onthe side of the movable frame connector 211 b. When an attracting forceacts on the mirror 230 in accordance with application of a predeterminedbias voltage to the electrodes 340 a to 340 d, the mirror 230 tiltsabout the movable frame pivot axis k and mirror pivot axis m so that theend in a direction (to be referred to as a “−a direction” hereinafter)reverse to the a direction in FIG. 50 approaches to the side of the base310 (this state will be referred to as “tilt in the −a direction”hereinafter). In this embodiment, individual control voltages areapplied to the electrodes 340 a to 340 d in this initial state, therebytilting the mirror 230.

In the mirror device 2 shown in FIG. 50, the moving distances of themovable frame connectors 211 a and 211 b and mirror connectors 221 a and221 b are freely set as needed in accordance with the position of themirror device 2 in the mirror array. The moving directions of themovable frame connectors 211 a and 211 b and mirror connectors 221 a and221 b, i.e., the positive/negative sign of the X and Y directions arefreely set as needed in accordance with the position of the mirrordevice 2 in the mirror array.

In the mirror device 2 shown in FIG. 50, the movable frame connectors211 a and 211 b and mirror connectors 221 a and 221 b simultaneouslymove in the Y direction to prevent the movable frame pivot axis andmirror pivot axis from passing through the gravity center G of themirror 230. Instead, one of the movable frame connectors 211 a and 211 band one of the mirror connectors 221 a and 221 b may move to prevent themirror pivot axis from passing through the gravity center G of themirror 230.

A mirror array according to this embodiment will be described next withreference to FIG. 51. FIG. 51 illustrates a state wherein apredetermined bias voltage is applied to the electrodes 340 a to 340 d.Each mirror device shown in FIG. 51 is in the state shown in FIG. 45,48, 49, or 50 viewed from the front side. That is, each mirror devicehas the mirror connectors 221 a and 221 b in the horizontal directionand the movable frame connectors 211 a and 211 b in the verticaldirection.

In a mirror array 900 according to this embodiment, the mirror devices 2described with reference to FIGS. 45, 48, 49, and 50 aretwo-dimensionally arranged in a matrix. The mirror array 900 correspondsto the mirror arrays 510 an 520 of the optical switch 600 shown in FIG.29. In the mirror array 900, at least one of the movable frameconnectors 211 a and 211 b and mirror connectors 221 a and 221 b isformed at a position that does not pass through the gravity center ofthe mirror 230 so that the mirror 230 reflects a light beam to a mirrorlocated at the center of the counter mirror array in the initial statewherein a uniform bias voltage is applied to the electrodes 340 a to 340d. In a mirror device 2 a located at the center of the mirror array 900,the movable frame connectors 211 a and 211 b and mirror connectors 221 aand 221 b are formed such that the movable frame pivot axis and mirrorconnectors pass through the gravity center of the mirror 230 whileintersecting each other at a right angle.

For example, in a mirror device 2 b adjacent to the mirror device 2 a inthe Y direction, the mirror connectors 221 a and 221 b move in the +Ydirection from the gravity center G such that the mirror 230 tilts inthe −Y direction in the initial state, like the mirror device 2 shown inFIG. 45. In a mirror device 2 c adjacent to the mirror device 2 b in theY direction, the mirror connectors 221 a and 221 b move in the Ydirection more than the mirror connectors 221 a and 221 b of the mirrordevice 2 b such that the mirror 230 tilts in the −Y direction more thanthe mirror 230 of the mirror device 2 b in the initial state.

In a or device 2 d adjacent to the mirror device 2 a in the X direction,the movable frame connectors 211 a and 211 b move in the +X directionfrom the gravity center G such that the mirror 230 tilts in the −Xdirection in the initial state, like the mirror device 2 shown in FIG.49. In a mirror device 2 e adjacent to the mirror device 2 d in the Xdirection, the movable frame connectors 211 a and 211 b move in the +Xdirection more than the movable frame connectors 211 a and 211 b of themirror device 2 d such that the mirror 230 tilts in the −X directionmore than the mirror 230 of the mirror device 2 d in the initial state.

In a mirror device 2 f adjacent to the mirror device 2 a in the X and Ydirections, i.e., in the direction of the arrow a in FIG. 50, the mirrorconnector 221 a moves in the +Y direction from the gravity center G suchthat the mirror 230 tilts in and −X and −Y directions, i.e., a direction(to be referred to as a “−a direction” hereinafter) reverse to the arrowa in FIG. 50 more than in the mirror device 2 a, like the mirror device2 shown in FIG. 48. In a mirror device 2 g adjacent to the mirror device2 f in the a direction, the mirror connector 221 a moves in the +Ydirection more than the mirror connector 221 a of the mirror device 2 fsuch that the mirror 230 tilts in the −a direction more than the mirror230 of the mirror device 2 f in the initial state.

In the mirror devices 2 f and 2 g, the movable frame connector 211 a maymove in the +X direction. In the mirror devices 2 f and 2 g, the movableframe connectors 211 a and 211 b and mirror connectors 221 a and 221 bmay move in the +X and +Y directions, respectively. In the mirrordevices 2 f and 2 g, the movable frame connector 211 a and mirrorconnector 221 a may move in the +Y and +X directions, respectively.

As described above, the positions of the movable frame connectors 211 aand 211 b and mirror connectors 221 a and 221 b are set in accordancewith the position of the mirror device in the mirror array 900. When abias voltage having a uniform magnitude is applied to the electrodes 340a to 340 d, the mirror 230 of each mirror device 2 of the mirror array900 tilts to reflect a received light beam to the mirror at the centerof the counter mirror array. This reflecting operation is the same asthat described in the ninth embodiment with reference to FIG. 30.

FIG. 30 is a sectional view taken along a line II-II in FIG. 51. FIG. 30schematically illustrates the sections of the mirrors 230 of theplurality of mirror devices 2 arranged in mirror arrays 510 and 520 eachincluding the mirror array 900. Each of mirrors 511 to 515 of the mirrorarray 510 and mirrors 521 to 525 of the mirror array 520 corresponds tothe mirror 230 included in the above-described mirror array 900. Themirror arrays 510 and 520 shown in FIG. 30 correspond to the mirrorarrays 510 and 520, respectively, of the optical switch 600 shown inFIG. 29.

As described above, in this embodiment, the movable frame connectors 211a and 211 b and mirror connectors 221 a and 221 b are provided such thatat least one of the movable frame pivot axis and mirror pivot axis doesnot pass through the gravity center of the mirror 230. Since the mirrortilts by a predetermined angle in the initial state, the mirror tiltangle can be small consequently. Accordingly, the driving voltage of themirror device and mirror array can be low.

The mirror device and mirror array according to this embodiment aremanufactured by the same manufacturing method as in the above-describedninth embodiment.

In this embodiment, in the step of forming, in the single-crystalsilicon layer, trenches conforming to the shapes of the frame portion210, movable frame connectors 211 a and 211 b, movable frame 220, mirrorconnectors 221 a and 221 b, and mirror 230, the movable frame connectors211 a and 211 b and mirror connectors 221 a and 221 b are formed whileshifting their positions in the above-described X and Y directionsdepending on the position in the mirror array.

The positions of the movable frame connectors 211 a and 211 b and mirrorconnectors 221 a and 221 b of each mirror device 2 of the thusmanufactured mirror array 900 are adjusted in accordance with itsposition in the mirror array 900. Each mirror device reflects a lightbeam to the mirror at the center of the counter mirror array uponapplying a predetermined bias voltage to the electrodes 340 a to 340 d.This allows to decrease the tilt angle of the mirror 230 of each mirrordevice 2. Additionally, since the mirror 230 tilts in the initial state,the operation range of the mirror 230 is small, and consequently, lowvoltage driving is possible.

FIG. 38 shows a modification of the mirror substrate 200. In thisembodiment, as described above, the movable frame connectors 211 a and211 b are provided in the first notches 222 a and 222 b formed in themovable frame 220. However, the movable frame connectors 211 a and 211 bneed not always exist there and may exist in third notches 224 a and 224b formed in the frame portion 210, as shown in FIG. 38. Even in themirror substrate 200, it is possible to reduce the tilt angle of themirror 230 by preventing the movable frame pivot axis and mirror pivotaxis from passing through the gravity center of the mirror 230.

The electrodes 340 a to 340 d need not always exist on the base 310 andmay exist on, e.g., a projecting portion provided on the base 310.Alternatively, the electrodes 340 a to 340 d may exist on the projectingportion and base 310.

In this embodiment, the mirror array 900 shown in FIG. 51 has 5×5 mirrordevices 2. However, the number of mirror devices 2 provided in themirror array 900 is not limited to 5×5 and can freely be set as needed.

The mirror device 2 and mirror array according to this embodiment areusable not only in an optical switch but also in a measurement device,display, and scanner. In this case, the projecting portion 320 andelectrodes 340 a to 340 d of the mirror device 2 are provided atarbitrary positions in accordance with the application purpose andspecifications.

In this embodiment, a bias voltage and displacement electrodes areapplied to the electrodes 340 a to 340 d. However, only the displacementvoltages may be applied.

As described above, according to this embodiment, since the mirror pivotaxis does not pass through the gravity center, the mirror tilts by apredetermined angle upon bias voltage application to the electrodes.Since the mirror is tilted by applying control voltages in this tiltedstate, the mirror tilt angle can be small. Since the tilt angle issmall, low voltage driving is possible.

14th Embodiment

The 14th embodiment of the present invention will be described next.

Although not illustrated in FIGS. 107 and 108, when a mirror arrayincludes a plurality of mirrors 8103 (mirror devices) two-dimensionallyintegrated, conventionally, interconnections to supply a voltage todriving electrodes 8003-1 to 8003-4 of a second mirror device pass nearthe mirror 8103 of a given mirror device (to be referred to as a firstmirror device hereinafter). Since a driving voltage is applied to theinterconnections, the mirror 8103 of the first mirror device alsoreceives an electrostatic force from the interconnections. Hence, a tiltangle θ of the mirror 8103 has a value shifted from the appropriateangle decided by the voltage applied to the driving electrodes 8003-1 to8003-4 of the first mirror device. In addition, the driving voltageapplied to the driving electrodes 8003-1 to 8003-4 of the second mirrordevice changes any time depending on the state of the optical switch.For this reason, with an influence from the interconnections to theplurality of second mirror devices, it is difficult to control the tiltangle of the mirror 8103 of the first mirror device.

Such interference from interconnections is negligible if the mirror 8103is excessively spaced apart from the interconnections. This increasesthe layout pitch of the mirrors 8103 and uneconomically increases thesize of the entire mirror array. In the optical switch, a pair of mirrorarrays facing each other exchange a light beam. If the mirror layoutpitch in the mirror array increases, the tilt angle θ required for eachmirror 8103 also increases to make manufacturing difficult. There is arequirement for arranging the plurality of mirrors 8103 close as much aspossible. If the interconnections can be led to the lower surface sideof a lower substrate 8001 by forming vertical holes in the lowersubstrate 8001 with the driving electrodes 8003-1 to 8003-4, it ispossible to suppress interference from the interconnections whilearranging the plurality of mirrors 8103 in the vicinity. However,leading the interconnections to the lower surface side of the lowersubstrate 8001 is technically difficult. That is, it is preferable inmanufacturing to two-dimensionally form the interconnections on thesurface of the lower substrate 8001. However, if the interconnectionsexist near the mirror 8103, interference of the interconnections to themirror 8103 is not negligible.

This embodiment has been made to solve the above-described problem andhas as its object to suppress an unexpected variation in the mirror tiltangle due to interference from interconnections in the neighborhood in amirror device and a mirror array including a plurality of mirror devicesarranged two-dimensionally.

In this embodiment, as shown in FIGS. 52 and 53, four driving electrodes1003-1 to 1003-4 are provided at the center of a lower substrate 1001made of single-crystal silicon. Supports 1004 of single-crystal siliconare provided on both sides of the upper surface of the lower substrate1001.

An upper substrate 1101 has an annular gimbal 1102 inside. A mirror 1103is provided inside the gimbal 1102. For example, a Ti/Pt/Au layer with athree-layered structure is formed on the upper surface of the mirror1103. Torsion springs 1104 connect the upper substrate 1101 to thegimbal 1102 at two 180° opposite points. Similarly, torsion springs 1105connect the gimbal 1102 to the mirror 1103 at two 180° opposite points.The X-axis passing through the pair of torsion springs 1104 and theY-axis passing through the pair of torsion springs 1105 intersect at aright angle. As a result, the mirror 1103 can pivot around the X- andY-axes each serving as a pivot axis. The upper substrate 1101, gimbal1102, mirror 1103, and torsion springs 1104 and 1105 are integrally madeof single-crystal silicon.

The structure of the lower substrate 1001 and the structure of the uppersubstrate 1101 shown in FIGS. 52 and 53 are separately manufactured. Theupper substrate 1101 is soldered to the supports 1004 so that the uppersubstrate 1101 bonds to the lower substrate 1001. In this mirror device,the mirror 1103 is grounded. A positive voltage is applied to thedriving electrodes 1003-1 to 1003-4 to generate an asymmetricalpotential difference between the driving electrodes 1003-1 to 1003-4. Anelectrostatic force attracts the mirror 1103 and causes it to pivot inan arbitrary direction.

In this embodiment, interconnections 1005-1 to 1005-4 are formed on theupper substrate 1101. The interconnections 1005-1 to 1005-4 connect tothe driving electrodes 1003-1 to 1003-4, respectively, to supply adriving voltage from a power supply (not shown) to the drivingelectrodes 1003-1 to 1003-4. Interconnections 1006 connect to thedriving electrodes (not shown) of other mirror devices formed on thesame substrates 1001 and 1101 as those of the mirror device shown inFIGS. 52 and 53 to supply a driving voltage to the driving electrodes.

In the mirror device shown in FIGS. 52 and 53, if the interconnections1005-1 to 1005-4 and 1006 run in parallel to the X- and Y-axes, theyhave strong influence on pivotal movement of the gimbal 1102 about theX-axis and the mirror 1103 about the Y-axis. Especially, theinterconnections 1005-1 to 1005-4 and 1006 readily influence on thegimbal 1102 outside the mirror 1103. This is because the gimbal 1102exists closer to the interconnections 1005-1 to 1005-4 and 1006 than themirror 1103.

On the other hand, if the interconnections 1005-1 to 1005-4 and 1006exist in a direction perpendicular to the X-axis, they hardly interferewith pivotal movement of the gimbal 1102 about the X-axis. Similarly, ifthe interconnections 1005-1 to 1005-4 and 1006 exist in a directionperpendicular to the Y-axis, they hardly interfere with pivotal movementof the mirror 1103 about the Y-axis.

In this embodiment, with emphasis on suppressing the influence of theinterconnections 1005-1 to 1005-4 and 1006 on the gimbal 1102 thatreadily receives the influence, the interconnections 1005-1 to 1005-4and 1006 are provided in the Y direction perpendicular to the pivot axisof the gimbal 1102. The influence on the pivotal movement of the mirror1103 is suppressed by increasing the distance between the mirror 1103and the interconnections 1005-1 to 1005-4 and 1006, as will be describedbelow.

When the accuracy of tilt angle control of the mirror 110 is about1/1000 of the total tilt angle, the influence of the interconnections1005-1 to 1005-4 and 1006 is allowable in the same or less degree. Theaccuracy of about 1/1000 of the total tilt angle corresponds to a beamposition accuracy of about 10 μm when 10×10 mirrors 1103 having adiameter of, e.g., 500 μm are arranged at a pitch of 1 mm.

The actual measurement result of the influence of the electrostaticforce from the interconnections on the mirror 1103 will be describedwith reference to FIGS. 54 and 55. Referring to FIG. 54, the abscissarepresents a horizontal distance h from an end of the mirror 1103 to aninterconnection, and the ordinate represents the shift of the tilt angleof the mirror 1103 generated by the electrostatic force from theinterconnection. The rotation spring rigidity of the torsion spring 1105is 2.4×10⁻⁹ Nm, the voltage applied to the interconnection is 80 V, awidth W of the interconnection is 9 μm, and a distance d between themirror 1103 and the driving electrodes 1003-1 to 1003-4 is 87.8 μm.

An actual measurement result obtained under these conditions is acharacteristic A1 in FIG. 54. Under actual use conditions, the drivingvoltage is, e.g., about 240 V at maximum. The characteristic A1 changesto a characteristic A2 in FIG. 54 when the driving voltage is 240 V. Inthe characteristic A2, the angle shift of the mirror 1103 is about ninetimes as compared to the characteristic A1. The characteristics A1 andA2 are obtained assuming that only one interconnection exists. When thedriving voltage of 240 V is applied to 12 interconnections (the totalinterconnection width W is 200 μm), the characteristic A1 changes to acharacteristic A3 in FIG. 54. In the characteristic A3, the angle shiftof the mirror 1103 is about 30 times as compared to the characteristicA1.

As is apparent from the characteristic A3 in FIG. 54, when thehorizontal distance h equals the distance d between the mirror 1103 andthe driving electrodes 1003-1 to 1003-4, the angle shift decreases byabout an order of magnitude as compared to the case of h=0. When h=2 d,the angle shift decreases by about two orders of magnitude as comparedto the case of h=0. When h=4 d, the angle shift decreases by about threeorders of magnitude. This result indicates that the influence of theinterconnections decreases to an effectively negligible level when theinterconnections 1005-1 to 1005-4 and 1006 are spaced apart from themirror 1103 by, e.g., about 4 d.

In this embodiment, therefore, an unexpected variation in the tilt angleof the mirror 1103 due to interference from the interconnections 1005-1to 1005-4 and 1006 can be suppressed by providing the interconnections1005-1 to 1005-4 and 1006 in the direction perpendicular to the pivotaxis of the gimbal 1102. This suppressing effect can further increasewhen the interconnections 1005-1 to 1005-4 and 1006 are spaced apartfrom the mirror 1103. In a uniaxial-pivot mirror device without anygimbal, the interconnections 1005-1 to 1005-4 and 1006 are provided in adirection perpendicular to the pivot axis of the mirror 1103.

As described above, according to this embodiment, in a uniaxial-pivotmirror device having mirror supported to pivot with respect to the uppersubstrate, interconnections connected to the driving voltages arearranged on the lower substrate perpendicularly to the pivot axis of themirror, thereby suppressing an unexpected variation in the mirror tiltangle due to interference from the interconnections. Additionally, sinceit is unnecessary to form interconnections on the lower surface side ofthe lower substrate, the variation in the mirror tilt angle can besuppressed while maintaining easy manufacturing.

According to this embodiment, in a biaxial-pivot mirror device that hasan annular gimbal supported to pivotal with respect to the uppersubstrate and a mirror supported to pivotal with respect to the gimbaland pivots about two axes that intersect at a right angle,interconnections connected to the driving voltages are arranged on thelower substrate perpendicularly to the pivot axis of the gimbal, therebysuppressing an unexpected variation in the mirror angle due tointerference from the interconnections. Additionally, since it isunnecessary to form interconnections on the lower surface side of thelower substrate, the variation in the mirror tilt angle can besuppressed while maintaining easy manufacturing.

15th Embodiment

In the 14th embodiment, the interconnections of a single mirror devicehave been described. In the 15th embodiment, the interconnections of amirror array including a plurality of mirror devices arrangedtwo-dimensionally will be described with reference to FIG. 56. The samereference numerals as in FIGS. 52 and 53 denote the same parts in FIG.56. Referring to FIG. 56, interconnections 1007 connect to the drivingelectrodes of each mirror device (the interconnections 1007 correspondto the interconnections 1005-1 to 1005-4 and 1006 in FIGS. 52 and 53).Electrode terminals 1008 for wire bonding connect to theinterconnections 1007.

In the mirror array shown in FIG. 56, the electrode terminals 1008connected to an external power supply are arranged at the outerperiphery of a rectangular mirror arrangement region 1201. Theinterconnections 1007 run from the electrode terminal 1008 toward thecenter. This realizes an efficient interconnection layout.

In this embodiment, two diagonal lines 1202 and 1203 divide the mirrorarrangement region 1201 into four divided regions 1204 to 1207. In twoarbitrary adjacent divided regions surrounded by two intersecting sidesof the mirror arrangement region 1201 and the diagonal lines 1202 and1203, the mirror devices are arranged while making the pivot axes (axespassing through pairs of torsion springs 1104) of gimbals 1102 intersectat a right angle. For example, the divided region 1204 has the gimbals1102 with their pivot axes set in the horizontal direction in FIG. 56.The divided region 1205 adjacent to the divided region 1204 has thegimbals 1102 with their pivot axes set in the vertical direction in FIG.56. In the divided regions 1204 to 1207, the interconnections 1007 runperpendicularly to the pivot axes of the gimbals 1102, as in the 14thembodiment.

In this way, this embodiment can suppress the unexpected variation inthe tilt angle of the mirror 1103 due to interference from theinterconnections 1007.

As described above, according to this embodiment, in a mirror arrayhaving a plurality of biaxial-pivot mirror devices arrangedtwo-dimensionally, two arbitrary adjacent divided regions surrounded bytwo intersecting sides of the rectangular mirror arrangement region andthe diagonal lines have the mirror devices including gimbals with pivotaxes intersecting at a right angle. In each divided region, theinterconnections connected the driving electrodes run on the lowersubstrate perpendicularly to the pivot axes of the gimbals. Thisstructure can suppress any unexpected variation in the mirror tilt angledue to interference from the interconnections.

16th Embodiment

The 16th embodiment of the present invention will be described next.This embodiment suppresses an unexpected variation in the mirror tiltangle due to interference from interconnections by arranging, at aposition closer to the interconnections than a mirror, a conductivemember equipotential to the mirror. The same names as in the 14thembodiment denote the same constituent elements in the 16th embodiment.

As shown in FIGS. 57 and 58, an insulating layer 1002 made of a siliconoxide film is formed on a lower substrate 101 of single-crystal silicon.Four driving electrodes 1003-1 to 1003-4 are provided on the insulatinglayer 1002 at the center of the lower substrate 1001. Supports 1004 ofsingle-crystal silicon are provided on both sides of the upper surfaceof the lower substrate 1001. Interconnections 1005-1 to 1005-4 connectto the driving electrodes 1003-1 to 1003-4, respectively, to supply adriving voltage from a power supply (not shown) to the drivingelectrodes 1003-1 to 1003-4. Interconnections 1006 connect to thedriving electrodes (not shown) of other mirror devices formed on thesame substrates 1001 and 1101 as those of the mirror device shown inFIGS. 57 and 58 to supply a driving voltage to the driving electrodes.

The upper substrate 1101 has an annular gimbal 1102 inside. A mirror1103 is provided inside the gimbal 1102. For example, a Ti/Pt/Au layerwith a three-layered structure is formed on the upper surface of themirror 1103. Torsion springs 1104 connect the upper substrate 1101 tothe gimbal 1102 at two 180° opposite points. Similarly, torsion springs1105 connect the gimbal 1102 to the mirror 1103 at two 180° oppositepoints. The X-axis passing through the pair of torsion springs 1104 andthe Y-axis passing through the pair of torsion springs 1105 intersect ata right angle. As a result, the mirror 1103 can pivot around the X- andY-axes each serving as a pivot axis. The upper substrate 1101, gimbal1102, mirror 1103, and torsion springs 1104 and 1105 are integrally madeof single-crystal silicon.

The structure of the lower substrate 1001 and the structure of the uppersubstrate 1101 shown in FIGS. 57 and 58 are separately manufactured. Theupper substrate 1101 is soldered to the supports 1004 so that the uppersubstrate 1101 bonds to the lower substrate 1001. In this mirror device,the mirror 1103 is grounded. A positive or negative voltage is appliedto the driving electrodes 1003-1 to 1003-4 to generate an asymmetricalpotential difference between the driving electrodes 1003-1 to 1003-4. Anelectrostatic force attracts the mirror 1103 and causes it to pivot inan arbitrary direction.

As described above, the upper substrate 1101, gimbal 1102, mirror 1103,and torsion springs 1104 and 1105 are integrally made of single-crystalsilicon. A predetermined potential (e.g., ground potential) is appliedto the mirror 1103 through the upper substrate 1101, torsion springs1104, gimbal 1102, and torsion springs 1105.

As a characteristic feature of this embodiment, a conductive memberequipotential to the mirror 1103 is arranged at a position closer to theinterconnections 1005-1 to 1005-4 and 1006 than the mirror 1103. Thisstructure suppresses any unexpected variation in the tilt angle of themirror 1103 due to interference from the interconnections 1005-1 to1005-4 and 1006. In this embodiment, the lower substrate (conductivesubstrate) 1001 of single-crystal silicon serves as the conductivemember equipotential to the mirror 1103. That is, the lower substrate1001 is equipotential to the mirror 1103, as shown in FIG. 59.

A distance d between the mirror 1103 and the driving electrodes 1003-1to 1003-4 is, e.g., about 90 μm. Since the thickness of the insulatinglayer 1002 that exists between the interconnections 1005-1 to 1005-4 and1006 and the lower substrate 1001 is smaller than the distance d, thelower substrate 1001 is closer to the interconnections 1005-1 to 1005-4and 1006 than the mirror 1103. For this reason, when the lower substrate1001 is equipotential to the mirror 1103, most of lines E of electricforce generated by applying a voltage V to the interconnection 1005-1terminate on the side of the lower substrate 1001, as shown in FIG. 59.This also applies to the remaining interconnections 1005-2 to 1005-4,although FIG. 59 shows only the lines of electric force of theinterconnection 1005-1, for the illustrative convenience. According tothis embodiment, it is possible to suppress any unexpected variation inthe tilt angle of the mirror 1103 due to interference from theinterconnections 1005-1 to 1005-4 and 1006.

When an insulating substrate is used as the lower substrate, the linesof electric force from the interconnections terminate on the mirror-sidesubstrate, resulting in stronger interference to the mirror. Even withsuch an insulating substrate, the same effect as in this embodiment isavailable if shielding interconnections equipotential to the mirror arearranged adjacent to the interconnections. However, the shieldinginterconnections consume an extra area so no wide spacing between theinterconnections and the mirror can be maintained.

As described above, according to this embodiment, the expected variationin the mirror tilt angle due to interference from the interconnectionscan be suppressed by arranging, at a position closer to theinterconnections than the mirror, a conductive member equipotential tothe mirror. In this embodiment, since it is unnecessary to forminterconnections on the lower surface side of the lower substrate, thevariation in the mirror tilt angle can be suppressed while maintainingeasy manufacturing. In this embodiment, since the lower substrate servesas the conductive member equipotential to the mirror, the conductivemember located closer to the interconnections than the mirror can easilybe obtained.

17th Embodiment

The 17th embodiment of the present invention will be described next. Thesame reference numerals as in FIGS. 57 and 58 denote the same parts inFIG. 60.

In this embodiment, a lower substrate 1001 serves as a conductive memberequipotential to a mirror 1103, as in the 16th embodiment. In addition,a conductive layer 1008 formed on an insulating layer 1007 oninterconnections 1005-1 to 1005-4 and 1006 also serves as a conductivemember equipotential to the mirror 1103.

The conductive layer 1008 electrostatically shields the interconnections1005-1 to 1005-4 and 1006 to enhance the effect of the first embodimentthat suppresses the unexpected variation in the tilt angle of the mirror1103 clue to interference from the interconnections 1005-1 to 1005-4 and1006.

To make the conductive layer 1008 equipotential to the mirror 1103, acontact hole 1009 is formed by partially removing an insulating layer1002 on the surface of the lower substrate 1001. This structure candirectly connect the conductive layer 1008 to the lower substrate 1001without routing an interconnection to make the conductive layer 1008equipotential to the mirror 1103. It is therefore easy to ensureelectrical connection to the isolated conductive layer 1008 that hasdifficulty in interconnection.

As described above, according to this embodiment, a conductive layerformed on an insulating layer on the interconnections serves as aconductive member equipotential to the mirror. This enhances the effectof suppressing the unexpected variation in the mirror tilt angle due tointerference from the interconnections.

18th Embodiment

The 18th embodiment of the present invention will be described next. Thesame reference numerals as in FIGS. 57 and 58 denote the same parts inFIG. 61.

In this embodiment, a lower substrate 1001 serves as a conductive memberequipotential to a mirror 1103, as in the 16th embodiment. In addition,a conductive wail-shaped member 1010 arranged between the mirror 1103and interconnections 1005-1 to 1005-4 and 1006 also serves as aconductive member equipotential to the mirror 1103.

The wall-shaped member 1010 shields the interconnections 1005-1 to1005-4 and 1006 from the mirror 1103 to enhance the effect of the 16thembodiment that suppresses the unexpected variation in the tilt angle ofthe mirror 1103 due to interference from the interconnections 1005-1 to1005-4 and 1006.

To make the wall-shaped member 1010 equipotential to the mirror 1103, acontact hole 1011 is formed by partially removing an insulating layer1002, as in the 17th embodiment. The lower substrate 101 is formed onthe contact hole 1011. This structure can easily make the wall-shapedmember 1010 equipotential to the mirror 1103.

As described above, according to this embodiment, a conductivewall-shaped member arranged between the mirror and the interconnectionsserves as a conductive member equipotential to the mirror. This enhancesthe effect of suppressing the unexpected variation in the mirror tiltangle due to interference from the interconnections.

In the 16th to 18th embodiments, increasing the distance between themirror 1103 and the interconnections 1005-1 to 1005-4 and 1006 canenhance the effect of the 16th to 18th embodiments. When the accuracy oftilt angle control of the mirror 1103 is about 1/1000 of the total tiltangle, the influence of the interconnections 1005-1 to 1005-4 and 1006is allowable in the same or less degree. The accuracy of about 1/1000 ofthe total tilt angle corresponds to a beam position accuracy of about 10μm when 10×10 mirrors 1103 having a diameter of, e.g., 500 μm arearranged at a pitch of 1 mm.

19th Embodiment

The 19th embodiment of the present invention be described next.

A conventional mirror device will be described first with reference toFIGS. 62 and 63. The same reference numerals as in FIGS. 107 and 108denote the same constituent elements in FIGS. 62 and 63.

A terrace-shaped projecting portion 8005 is provided at the center of alower substrate 8001 of single-crystal silicon. Four driving electrodes8003-1 to 8003-4 are provided on an insulating layer 8002 made of asilicon oxide film on the four corners of the projecting portion 8005and the lower substrate 8001 continuous from the four corners. Supports8004 of single-crystal silicon are provided on both sides of the uppersurface of the lower substrate 8001.

An upper substrate 8101 has an annular gimbal 8102 inside. A mirror 8103is provided inside the gimbal 8102. For example, a Ti/Pt/Au layer with athree-Layered structure is formed on the upper surface of the mirror8103. Torsion springs 8104 connect the upper substrate 3101 to thegimbal 8102 at two 180° opposite points. Similarly, torsion springs 8105connect the gimbal 8102 to the mirror 8103 at two 180° opposite points.The X-axis passing through the pair of torsion springs 8104 and theY-axis passing through the pair of torsion springs 8105 intersect at aright angle. As a result, the mirror 8103 can pivot around the X- andY-axes each serving as a pivot axis. The upper substrate 8101, gimbal8102, mirror 8103, and torsion springs 8104 and 8105 are integrally madeof single-crystal silicon.

The structure of the lower substrate 8001 and the structure of the uppersubstrate 8101 shown in FIGS. 62 and 63 are separately manufactured. Theupper substrate 8101 is soldered to the supports 8004 so that the uppersubstrate 8101 bonds to the lower substrate 8001. In this mirror device,the mirror 8103 is grounded. A positive voltage is applied to thedriving electrodes 8003-1 to 8003-4 to generate an asymmetricalpotential difference between the driving electrodes 8003-1 to 8003-4. Anelectrostatic force attracts the mirror 8103 and causes it to pivot inan arbitrary direction.

In the mirror device shown in FIGS. 62 and 63, the relationship betweena driving voltage V applied to the driving electrodes 8003-1 to 8003-4and a tilt angle θ of the mirror 8103 is nonlinear. Especially, when thetilt angle θ increases, the change in the tilt angle θ with respect tothe driving voltage V abruptly increases. Finally, dθ/dV becomesinfinite to generate an unstable state called pull-in or snap-downwherein the driving electrodes 8003-1 to 8003-4 attract the mirror 8103.A pull-in angle is about ⅓ the angle made by the mirror 8103 and thedriving electrodes 8003-1 to 8003-4 when no driving voltage is applied.FIG. 64 shows an example of a driving voltage vs. tilt anglecharacteristic curve. Referring to FIG. 64, θp is a pull-in angle, andVp is a pull-in voltage to give the pull-in angle θ_(p).

In the mirror device shown in FIGS. 62 and 63, the torsion springs 8104and 8105 support the mirror 8103 to make it freely pivot. The torsionsprings 8104 and 8105 ideally exhibit low spring rigidity only in thepivot direction and infinite rigidity for the remaining displacements.Actually, to obtain low spring rigidity in the pivot direction, thespring rigidity in the vertical direction and that in theexpansion/contraction direction must also be low. Hence, if a moment tomake the mirror 8103 pivot is generated by applying a voltage to thedriving electrodes 8003-1 to 8003-4, the mirror 8103 not only pivots butalso approaches the side of the driving electrodes 8003-1 to 8003-4.

At the same driving voltage, the tilt angle θ of the mirror 8103 islarger when the mirror 8103 sinks to the side of the driving electrodes8003-1 to 8003-4 and pivots than when the mirror 8103 pivots withoutmoving its gravity center position, i.e., pivotal center. This isbecause the electrostatic force increases in inverse proportion to thesecond power of the distance between the mirror 8103 and the drivingelectrodes 8003-1 to 8003-4. If the mirror 8103 sinks, the increase inthe tilt angle θ of the mirror 8103 with respect to the driving voltageexhibits larger nonlinearity than a case wherein the pivotal center isfixed. FIG. 65 shows an example of a driving voltage vs. tilt anglecharacteristic curve when the mirror 8103 sinks and pivots. The solidline in FIG. 65 indicates a characteristic when the mirror 8103 sinksand pivots. The broken line indicates a characteristic when the mirror8103 does not sink (characteristic in FIG. 64).

As is apparent from FIG. 65, when the mirror 8103 sinks, thenonlinearity of the tilt angle θ with respect to the driving voltage Vis more conspicuous. The pull-in angle θ_(p) and pull-in voltage Vp alsodecrease. The torsion spring structure which can prevent the mirror 8103from sinking is free from the above-described problem. As describedabove, it is difficult in the actual design to form the torsion springs8104 and 8105 that are soft only in the pivot direction of the mirror8103 and rigid in the sinking direction of the mirror 8103.

This embodiment has been made to solve the above-described problem andhas as its object to improve the nonlinear response of the mirror tiltangle with respect to the driving voltage in a mirror device.

The embodiment of the present invention will be described below withreference to the accompanying drawings. A terrace-shaped projectingportion 1305 is provided at the center of a lower substrate 1301 ofsingle-crystal silicon. Four driving electrodes 1303-1 to 1303-4 areprovided on an insulating layer 1302 made of a silicon oxide film on thefour corners of the projecting portion 1305 and the lower substrate 1301continuous from the four corners. Supports 1304 of single-crystalsilicon are provided on both sides of the upper surface of the lowersubstrate 1301.

An upper substrate 1401 has an annular gimbal 1402 inside. A mirror 1403is provided inside the gimbal 1402. For example, a Ti/Pt/Au layer with athree-layered structure is formed on the upper surface of the mirror1403. Torsion springs 1404 connect the upper substrate 1401 to thegimbal 1402 at two 180° opposite points. Similarly, torsion springs 1405connect the gimbal 1402 to the mirror 1403 at two 180° opposite points.The X-axis passing through the pair of torsion springs 1404 and theY-axis passing through the pair of torsion springs 1405 intersect at aright angle. As a result, the mirror 1403 can pivot around the X- andY-axes each serving as a pivot axis. The upper substrate 1401, gimbal1402, mirror 1403, and torsion springs 1404 and 1405 are integrally madeof single-crystal silicon.

The structure of the lower substrate 1301 and the structure of the uppersubstrate 1401 shown in FIGS. 66 and 67 are separately manufactured. Theupper substrate 1401 is soldered to the supports 1304 so that the uppersubstrate 1401 bonds to the lower substrate 1301. In this mirror device,the mirror 1403 is grounded. A positive voltage is applied to thedriving electrodes 1303-1 to 1303-4 to generate an asymmetricalpotential difference between the driving electrodes 1303-1 to 1303-4. Anelectrostatic force attracts the mirror 1403 and causes it to pivot inan arbitrary direction.

In this embodiment, to prevent the mirror 1403 from sinking, a pivot(column) 1306 to support the pivotal center of the mirror 1403 is formedon the upper surface of the projecting portion 1305. The pointed pivot1306 arranged at the position of the pivotal center of the mirror 1403fixes the pivotal center of the mirror 1403, prevents the mirror 1403from sinking, and allows the mirror 1403 to pivot up to the appropriatepull in angle.

In this embodiment, the pivot 1306 prevents the mirror 1403 fromsinking. For this reason, the spring rigidity of the torsion springs1404 and 1405 in the sinking direction (to the tower side of FIG. 67)can be low. The spring rigidity in the pivot direction (the direction ofthe arrow in FIG. 67) can also be low. As a result, the driving voltagenecessary for making the mirror 1403 pivot can be lower than before. Thelower substrate 1301, projecting portion 1305, and pivot 1306 areintegrally made of single-crystal silicon. The pivot 1306 can be formedby, e.g., anisotropic etching of silicon or high aspect ratio structureformation by RIE.

In this embodiment, a predetermined bias voltage Vb is applied from apower supply 1501 to the four divided driving electrodes 1303-1 to1303-4, as shown in FIG. 68. As before, the upper substrate 1401, gimbal1402, mirror 1403, and torsion springs 1404 and 1405 are integrally madeof single-crystal silicon. A ground potential is applied to the mirror1403 through the upper substrate 1401, torsion springs 1404, gimbal1402, and torsion springs 1405. When only the bias voltage Vb is appliedto the driving electrodes 1303-1 to 1303-4, the mirror 1403 maintainsthe equilibrium state (θ=0) without pivoting.

A method of making the mirror 1403 pivot from this equilibrium statewill be described. Uniaxial pivot will be explained here for thedescriptive convenience. Biaxial pivot will be described later. To makethe mirror 1403 pivot about, e.g., the Y-axis, Vb+Va is applied to, ofthe driving electrodes 1303-1 to 1303-4, the driving electrodes (to bereferred to as positive-side driving electrodes hereinafter) 1303-1 and1303-2 close to the mirror 1403, and Vb+Vc is applied to the drivingelectrodes (to be referred to as negative-side driving electrodeshereinafter) 1303-3 and 1303-4 far from the mirror 1403, as shown inFIG. 69. Let V be the driving voltage necessary for obtaining the tiltangle θ of the mirror 1403. In this case, the voltages Va and Vc appliedtogether with the bias voltage Vb are Va=V and Vc=−V. The bias voltageVb and driving voltage V have a relationship given by V<Vb.

In this embodiment, when the bias voltage Vb is applied, and in thisstate, a voltage difference is generated between the positive-sidedriving electrodes and the negative-side driving electrodes, the mirror1403 can pivot, and the linear response of the tilt angle θ with respectto the driving voltage V can improve. FIG. 70 shows an example of adriving voltage vs. tilt angle characteristic curve in the mirror deviceof this embodiment. According to this embodiment, an almost linearresponse is available in a relatively wide range from θ=0 to an angleclose to the pull-in angle θ_(p). If no bias voltage Vb is applied, themirror 1403 does not largely tilt until application of a high voltage,as shown in FIGS. 64 and 65. The mirror 1403 abruptly pivots then andreaches the pull-in state. On the other hand, when the bias voltage Vbis applied, as in this embodiment, the mirror 1403 pivots in proportionto the voltage difference between the positive-side driving electrodesand the negative-side driving electrodes. Hence, the pivotcontrollability of the mirror 1403 can improve.

The reason why the linear response of the tilt angle θ with respect tothe driving voltage can be improved by applying the bias voltage Vb willbe described below. The tilt angle θ of the mirror 1403 increases inaccordance with the voltage applied between the mirror 1403 and thedriving electrodes 1303-1 to 1303-4. The balance between the restoringforce of the torsion springs 1404 and 1405 by a spring rigidity k ofrotation of the torsion springs 1404 and 1405 and the electrostaticforce from the driving electrodes 1303-1 to 1303-4 decides the tiltangle θ.

Let M_(S) be the moment by the restoring force of the torsion springs1404 and 1405, and M_(E) be the moment by the electrostatic force. Themoments M_(S) and M_(E) are given byM _(S) =−kθ  (2)M _(E) =M _(E)(V,θ)  (3)The moment M_(E) by the electrostatic force depends on the shape of thedriving electrodes 1303-1 to 1303-4 and is therefore represented by thefunction M_(E)(V, θ). The spring rigidity k depends on the shape of thetorsion springs 1404 and 1405.

For the descriptive convenience, assume that a positive-side drivingelectrode 1303 a and a negative-side driving electrode 1303 b arearranged on slanting surfaces with an angle θa to be bilaterallysymmetrical with respect to a vertical line L that passes through thepivotal center of the mirror 1403, as shown in FIG. 71. The edges ofeach of the positive-side driving electrode 1303 a and negative-sidedriving electrode 1303 b are spaced apart from the vertical line L bydistances x1 and x2. The mirror 1403 pivots about the Y-axis(perpendicular to the drawing surface of FIG. 71). Pivotal movementabout the X-axis is neglected.

Let Vb be the bias voltage, and V be the driving voltage. A voltage(Vb−V) is applied to the negative-side driving electrode 1303 b. Avoltage (Vb+V) is applied to the positive-side driving electrode 1303 a.At this time, the moment M_(E) by the electrostatic force from thedriving electrodes 1303 a and 1303 b is given by

$\begin{matrix}{M_{E} = {{\frac{1}{2}{ɛ\left( {{Vb} + V} \right)}^{2}{\int_{x\; 1}^{x\; 2}\frac{\mathbb{d}x}{\left( {{\theta\; a} - \theta} \right)^{2}x}}} + {\frac{1}{2}{ɛ\left( {{Vb} - V} \right)}^{2}{\int_{{- x}\; 1}^{{- x}\; 2}\frac{\mathbb{d}x}{\left( {{\theta\; a} + \theta} \right)^{2}x}}}}} & (4)\end{matrix}$

En equation (4), the first term on the right-hand side indicates themoment by the positive-side driving electrode 1303 a, and the secondterm on the right-hand side indicates the moment by the negative-sidedriving electrode 1303 b. Equation (4) can be rewritten to

$\begin{matrix}{M_{E} = {\frac{1}{2}{ɛ\left\lbrack {\frac{\left( {{Vb} + V} \right)^{2}}{\left( {{\theta\; a} - \theta} \right)^{2}} - \frac{\left( {{Vb} - V} \right)^{2}}{\left( {{\theta\; a} + \theta} \right)^{2}}} \right\rbrack}\ln\frac{x\; 2}{x\; 1}}} & (5)\end{matrix}$

Assuming θ/θa<<1, equation (5) can be approximated to

$\begin{matrix}{M_{E} = {2{ɛ\left( {{\frac{Vb}{\theta\; a^{2}}V} + {\frac{{Vb}^{2}}{\theta\; a^{3}}\theta}} \right)}\ln\frac{x\; 2}{x\; 1}}} & (6)\end{matrix}$

The first term in the parentheses of equation (6) represents that themoment M_(E)is proportional to the first power of the voltage V. Sinceθ<θa/3 and V<Vb, the second term in the parentheses of equation (6) hasa value so small that it is negligible as compared to the first term ina relatively wide angle range. The balance between the moment by theelectrostatic force and the restoring force of the torsion springs 1404and 1405 decides the tilt angle θ of the mirror 1403.

Hence, we obtainM _(S) +M _(E)=0  (7)From equations (2) and (6), we obtain

$\begin{matrix}{{{{- k}\;\theta} + {2{ɛ\left( {{\frac{Vb}{\theta\; a^{2}}V} + {\frac{{Vb}^{2}}{\theta\; a^{3}}\theta}} \right)}\ln\frac{x\; 2}{x\; 1}}} = 0} & (8)\end{matrix}$

Equation (8) can be approximated to

$\begin{matrix}{\theta \cong {\frac{2ɛ}{k}\frac{Vb}{\theta\; a^{2}}V\;\ln\frac{x\; 2}{x\; 1}}} & (9)\end{matrix}$

As is apparent from equation (9), when the bias voltage Vb is applied,the tilt angle) of the mirror 1403 linearly responds to the voltage V.

In a comparative case without application of the bias voltage Vb, themoment M_(E) by the electrostatic force is given by

$\begin{matrix}{M_{E} = {\frac{1}{2}ɛ\; V^{2}{\int_{x\; 1}^{x\; 2}\mspace{7mu}\frac{\mathbb{d}x}{\left( {{\theta\; a} - \theta} \right)^{2}x}}}} & (10)\end{matrix}$

Equation (10) can be rewritten to

$\begin{matrix}{M_{E} = {\frac{1}{2}ɛ\frac{V^{2}}{\left( {{\theta\; a} - \theta} \right)^{2}}\;\ln\frac{x\; 2}{x\; 1}}} & (11)\end{matrix}$

Assuming θ/θa<<1, equation (11) is approximated. From the approximateexpression and equations (2) and (7), we obtain

$\begin{matrix}{\theta \cong {\frac{1}{2k}ɛ\frac{V^{2}}{\theta\; a^{2}}\ln\frac{x\; 2}{x\; 1}}} & (12)\end{matrix}$

As is apparent from equation (12), when no bias voltage Vb is applied,the tilt angle θ of the mirror 1403 is proportional to the second powerof the voltage V even in a small angle range.

The bias voltage Vb has a clear upper limit value. This is because whenthe bias voltage Vb has a predetermined value or more, the state of themirror 1403 is unstable even at θ=0 and cannot be restored. Thisphenomenon indicates that the differential value of the sum of themoment by the restoring force of the torsion springs 1404 and 1405 andthe moment by the electrostatic force when θ=0 is 0 or more. That is,the mirror 1403 cannot be restored to the state when θ=0. The mirror1403 freely pivots in a direction in which the tilt angle θ increases.Hence, the condition of the bias voltage Vb is obtained from

$\begin{matrix}{{\frac{\mathbb{d}M_{E}}{\mathbb{d}\theta} + \frac{\mathbb{d}M_{S}}{\mathbb{d}\theta}} < 0} & (13)\end{matrix}$

That is, the bias voltage Vb satisfies dM_(S)/dθ<k. In the electrodestructure shown in FIG. 71, we obtain

$\begin{matrix}{\left( {\frac{\mathbb{d}}{\mathbb{d}\theta}\left\lbrack {{{- k}\;\theta} + {2{ɛ\left( {{\frac{Vb}{\theta\; a^{2}}V} + {\frac{{Vb}^{2}}{\theta\; a^{3}}\theta}} \right)}\ln\frac{x\; 2}{x\; 1}}} \right\rbrack} \right)_{{V = 0},{\theta = 0}} < 0} & (14)\end{matrix}$

Hence, we obtain

$\begin{matrix}{{Vb} < \sqrt{\frac{k\;\theta\; a^{3}}{2{ɛln}\frac{x\; 2}{x\; 1}}}} & (15)\end{matrix}$

As described above, application of the bias voltage Vb improves thelinear response of the tilt angle θ of the mirror 1403 with respect tothe driving voltage V. However, application of the bias voltage Vb tothe driving electrodes indicates that the driving electrode sideattracts the mirror 1403. When only the torsion springs 1404 and 1405support the mirror 1403, the mirror 1403 sinks so it is impossible toexpect improvement of the linear response. In extreme cases, onlypull-in to the driving electrodes occurs. To the contrary, in thisembodiment, the pivot 1306 is arranged at the position of the pivotalcenter of the mirror 1403 to prevent the mirror 1403 from sinking evenwhen the bias voltage Vb is applied. Hence, the linear response of thetilt angle θ with respect to the voltage V can improve.

Even in driving by applying the bias voltage Vb, the pull-in anglerarely changes from that without application of the bias voltage Vb.Hence, the driving method by applying the bias voltage Vb to the drivingelectrodes can improve the linear response of the tilt angle θ withrespect to the voltage V without decreasing the rotatable angle of themirror 1403 or increasing the load on the power supply 1501.

The driving method for biaxial pivot will be described finally. As shownin FIG. 72, in biaxial pivot, Vb+V1, Vb+V2, Vb+V3, and Vb+V4 are appliedto the four divided driving electrodes 1303-1, 1303-2, 1303-3, and1303-4, respectively.

To obtain the driving voltages V1 to V4 necessary for an arbitrary tiltangle of the mirror 1403, the electrostatic capacitance between themirror 1403 and the driving electrodes 1303-1 to 1303-4 is calculated onthe basis of the positional relationship between the mirror 1403 and thedriving electrodes 1303-1 to 1303-4. An electrostatic force at the tiltangle is calculated from the change in electrostatic capacitance for asmall mirror tilt angle. The moment by the electrostatic force isobtained from this value. The balance between this moment and therestoring force of the torsion springs 1404 and 1405 decides the thetilt angle of the mirror 1103.

To calculate the electrostatic capacitance between the mirror 1403 andthe driving electrodes 1303-1 to 1303-4, it is necessary to analyze theelectromagnetic field by using, as the boundary condition, thepositional relationship between the mirror 1403 and the drivingelectrodes 1303-1 to 1303-4. Normally, numerical analysis using afinite-element method is executed. Numerical analysis using afinite-element method is disclosed in, e.g., a reference “M. Fischer etal., “Electrostatically deflectable polysilicon micromirrors-dynamicbehavior and comparison with the results from FEM modeling with ANSYS”,Sensors and Actuators, Vol. A67, pp. 89-95, 1998”, or a reference “M.Urano et al., “Novel Fabrication Process and Structure of aLow-Voltage-Operation Micromirror Array for Optical MEMS Switches”,Technical digest of Electron Device Meeting (IEDM'03), 8-10, December2003”.

A table (storage unit) 1502 stores in advance the thus obtainedrelationship between the X- and Y-direction tilt angles of the mirror1403 and the driving voltages V1 to V4. A control circuit (acquisitionunit) 1503 acquires the values of the driving voltages V1 to V4corresponding to a desired tilt angle of the mirror 1403 from the table1502 and sets the values in the power supply 1501. The power supply 1501applies the set driving voltages V1 to V4 to the driving electrodes1303-1 to 1303-4 together with the predetermined bias voltage Vb.

The method of setting the bias voltage Vb will be described next. Asdescribed above, when a voltage of a certain value or more is applied tothe driving electrodes of the mirror device, an unstable state calledpull-in or snap-down occurs. To prevent this, it is necessary to limitthe voltage to be applied to the driving electrodes, i.e., set themaximum value (to be referred to as an applied voltage maximum valuehereinafter) of the applied voltage represented by the sum of thedriving voltage V and the bias voltage Vb. As already described above,the mirror tilt angle changes depending on the value of the drivingvoltage. Unless the value of the bias voltage Vb is appropriately set,the posture displacement amount of the mirror is limited, and thedesired mirror tilt angle cannot be obtained. For this reason, in thisembodiment, the bias voltage Vb is set as follows.

For example, in FIG. 73, the driving electrodes 1303-1 and 1303-3 aresymmetrical about the X-axis, and the driving electrodes 1303-2 and1303-4 are symmetrical about the Y-axis. In this case, to make themirror 1403 (not shown) arranged above the driving electrodes 1303-1 to1303-4 pivot about the Y-axis, Vb+Va is applied to, of the drivingelectrodes 1303-1 to 1303-4, the positive-side driving electrode 1303-1,and Vb+Vc is applied to the negative-side driving electrode 1303-3. LetVy be the driving voltage necessary for obtaining a tilt angle θy of themirror 1403. In this case, the voltages Va and Vc applied together withthe bias voltage Vb are Va=Vy and Vc=−Vy. Similarly, to make the mirror1403 pivot about the X-axis, Vb+Vd is applied to the positive-sidedriving electrode 1303-2, and Vb+Ve is applied to the negative-sidedriving electrode 1303-4. Let Vx be the driving voltage necessary forobtaining a tilt angle θx of the mirror 1403. In this case, the voltagesVd and Ve are Vd=Vx and Ve=−Vx.

FIG. 74 shows a measurement result of the tilt angle θy of the mirror1403 when the driving voltage Vy is changed by setting the appliedvoltage maximum value to 140 [V] and the driving voltage Vx to 0 [V] forthe driving electrodes shown in FIG. 73. A solid line a in FIG. 74indicates the relationship between the driving voltage Vy and the tiltangle θy when the bias voltage Vb is 0 [V]. Solid lines b to l alsoindicate the relationship between the driving voltage Vy and the tiltangle θy when the bias voltage Vb changes in steps of 10 [V].

As shown in FIG. 74, when the bias voltage Vb changes, the tilt angle θyalso changes. Especially, when the bias voltage Vb is about ½ theapplied voltage maximum value, as indicated by the solid lines i and h(Vb=70, 50 [V]), the tilt angle θy maximizes. Hence, in this embodiment,the bias voltage Vb is set to about ½ the applied voltage maximum value.This can increase the tilt angle of the mirror 1403.

The table 1502 stores the thus set bias voltage Vb in advance. Thecontrol circuit 1503 acquires the values of the driving voltages V1 toV4 corresponding to a desired tilt angle of the mirror 1403 and the biasvoltage Vb from the table 1502 and sets the values in the power supply1501. The power supply 1501 applies the set driving voltages V1 to V4 tothe driving electrodes 1303-1 to 1303-4 together with the bias voltageVb.

FIG. 74 explains the relationship between the driving voltage Vy and thetilt angle θy. This also applies to the relationship between the drivingvoltage Vy and the tilt angle θy. When the value of the bias voltageapplied to the driving electrodes is about ½ the applied voltage maximumvalue, the tilt angle can increase not only in the mirror device thatpivots about two axes, as shown in FIG. 73, but also in a mirror devicethat pivots about one axis or a mirror device that is translated.

As described above, according to this embodiment, it is possible toimprove the linear response of the mirror tilt angle with respect to thedriving voltage by forming a pivot on the lower substrate to support thepivotal center of a mirror and applying the same bias voltage to theplurality of driving electrodes facing the mirror.

According to this embodiment, a storage means stores in advance therelationship between the tilt angle and the driving voltages applied tothe driving electrodes. The values of the driving voltages necessary forobtaining a desired or tilt angle are acquired from the storage meansfor the respective driving electrodes, thereby individually deciding thevoltages to be applied to the respective driving electrodes.

According to this embodiment, the mirror tilt angle can increase whenthe bias voltage applied to the driving electrodes is about ½ themaximum voltage value applicable to the driving electrodes.

20th Embodiment

The 20th embodiment of the present invention will be described below.This embodiment has as its object to obtain a state wherein a largerpivot angle of a mirror is obtained by low-voltage driving withoutincreasing the cost.

FIG. 75 mainly partially illustrates a mirror device as a constituentunit of a mirror array. In a mirror array, mirror devices shown in FIG.75 are arrayed two-dimensionally in a square pattern. The mirror devicecomprises a mirror substrate 1500 with a mirror and an electrodesubstrate 1530 with electrodes. The mirror substrate 1500 and electrodesubstrate 1530 are arranged in parallel. FIG. 75 illustrates a surfaceof the mirror substrate 1500 facing the electrode substrate 1530, i.e.,a state different from the perspective view of FIG. 11.

The mirror substrate 1500 comprises a plate-shaped base 1501,ring-shaped movable frame 1502, and disk-shaped mirror 1503. The base1501 has an opening having an almost circular shape when viewed from theupper side. The movable frame 1502 is arranged in the opening of thebase 1501 and connected to the base 1501 by a pair of connectors 1501 aand 1501 b. The movable frame 1502 also has an opening having an almostcircular shape when viewed from the upper side. The mirror 1503 isarranged in the opening of the movable frame 1502 and connected to themovable frame 1502 by a pair of mirror connectors 1502 a and 1502 b. Aframe portion 1504 surrounding the movable frame 1502 and mirror 1503 isformed at the periphery of the base 1501. The frame portion 1504 isfixed to the base 1501 through an insulating layer 1505.

The connectors 1501 a and 1501 b including zigzag torsion springs andprovided in the notches of the movable frame 1502 connect the base 1501to the movable frame 1502. This structure makes the movable frame 1502connected to the base 1501 pivotable about a pivot axis (movable framepivot axis) passing through the connectors 1501 a and 1501 b. The mirrorconnectors 1502 a and 1502 b including zigzag torsion springs andprovided in the notches of the movable frame 1502 connect the movableframe 1502 to the mirror 1503. This structure makes the mirror 1503connected to the movable frame 1502 pivotable about a pivot axis (mirrorpivot axis) passing through the mirror connectors 1502 a and 1502 b. Inthe structural example shown in FIG. 75, the movable frame pivot axisand mirror pivot axis intersect each other at a right angle.

The electrode substrate 1530 has a projecting portion 1536 and ribstructures 1531 provided at the periphery of the projecting portion1536. The projecting portion 1536 includes a third terrace 1532 having atruncated pyramidal shape, a second terrace 1533 formed on the uppersurface of the third terrace 1532 and having a truncated pyramidalshape, a first terrace 1534 formed on the upper surface of the secondterrace 1533 and having a truncated pyramidal shape, and a pivot 1535formed on the upper surface of the first terrace 1534 and having atruncated pyramidal shape.

The upper surface of the electrode substrate 1530 including the outersurface of the projecting portion 1536 has a common electrode 1541 thatis integrally formed in, e.g., a circle concentric to the mirror 1503 onthe counter mirror substrate 1500. Interconnection 1537 are formedaround the projecting portion 1536 on the electrode substrate 1530. Theelectrode 1541 connects to the interconnections 1537 through a lead1542. The common electrode 1541 need only be formed as an integral metalfilm in a region including the region of the projecting portion 1536.Hence, no high fabrication accuracy is necessary for formation of thecommon electrode 1541. It is therefore possible to easily form thecommon electrode 1541 even in the projecting portion 1536 with largestep differences without any complex process. For example, in thephotolithography process of forming a fine mask pattern for theinterconnections 1537 by exposure based on a focal point, a mask patternfor the common electrode 1541 based on a largely different focal pointcan be formed simultaneously.

The mirror substrate 1500 shown in FIG. 75 also comprises drivingelectrodes 1503 a to 1503 d on the surface of the mirror 1503. Thedriving electrodes are symmetry with respect to the center of the mirror(mirror structure) 1503. The mirror 1503 on the mirror substrate 1500faces the common electrode 1541 on the counter electrode substrate 1530.A mirror device is formed by joining the lower surface of the base 1501to the upper surfaces of the rib structures 1531. In the mirror deviceshown in FIG. 75, the driving electrodes 1503 a to 1503 d formed on themirror 1503 face the common electrode 1541 formed on the electrodesubstrate 1530.

In the mirror device shown in FIG. 75, for example, the common electrode1541 is grounded. A positive or negative voltage is applied to thedriving electrodes 1503 a to 1503 d to generate an asymmetricalpotential difference between them. The mirror 1503 approaches the sideof the common electrode 1541 due to the generated electrostatic forceand pivots in an arbitrary direction. A power supply outside the mirrordevice applies the positive or negative voltage.

The projecting portion 1536 formed into a staircase shape with aplurality of terraces has the common electrode 1541. This allows todecrease the distance between the common electrode 1541 and the drivingelectrodes 1503 a to 1503 d without sacrificing a tilt angle θ of themirror 1503. Consequently, the mirror device shown in FIG. 75 can attaina large pull-in angle of the mirror 1503 and low-voltage driving. At thetilt angle θ equal to or more than the pull-in angle, it is impossibleto statically stably control the mirror 1503. If the tilt angle θ isequal to or larger than the pull-in angle, the electrostatic forcesurpasses the restoring force of the connectors so that the mirror 1503contacts the side of the electrode substrate 1530. The mirror deviceshown in FIG. 75 allows to easily form the common electrode 1541although the projecting portion 1536 has large step differences, asdescribed above. Hence, the tilt angle of the projecting portion 1536can easily be larger than before. As a result, in the mirror deviceshown in FIG. 75, the tilt angle of the mirror 1503 can be Large thanbefore.

Conventionally, to attain a large pull-in angle and low-voltage driving,a projecting portion 8320 is provided on an electrode substrate 8301,and electrodes are formed on the slanting surfaces of the projectingportion 8320, as shown in FIG. 11. On the other hand, interconnections8370 are formed on a base 8310 of the electrode substrate 8301. Informing a pattern for the electrodes or interconnections, the lowerlimit, the lower limit of focus of the exposure apparatus is set to theinterconnections 8370 having a finer pattern than the electrodes,thereby ensuring the accuracy of the formed pattern. However, the depthof field of the exposure apparatus is limited, and the difference ofelevation of the projecting portion 8320 must be limited to 50 to 70 μmor less.

However, to attain a larger pull-in angle and lower-voltage driving, thedifference of elevation of the projecting portion 8320 is preferablylarger and, more specifically, 100 μm or more. This state can beobtained by using a special exposure apparatus with a wide focus rangeor executing exposure a plurality of number of times for each step.However, a special exposure apparatus is expensive. An increase in thenumber of steps leads to an increase in the process cost. Hence, thecost increases conventionally to obtain the above-described arrangement.

The mirror device according to this embodiment comprises a commonelectrode formed on an electrode substrate, a mirror structure that ispivotally arranged apart above the electrode substrate while facing thecommon electrode, and a plurality driving electrodes provided on asurface of the mirror structure facing the common electrode. Forexample, when the common electrode is grounded, and a positive ornegative voltage is applied to a driving electrode, the mirror structurepivots due to the generated electrostatic force.

In the mirror device, an almost conical projecting portion is formed onthe electrode substrate, and at least part of the common electrode isformed on the projecting portion. The mirror structure is arranged topivot about a pivot axis passing through the center of the mirrorstructure. The plurality of driving electrodes may be symmetrical withrespect to the center of the mirror structure. The mirror device alsocomprises rib structures provided on the electrode substrate around thecommon electrode, and a mirror substrate to which the mirror structurepivotally connects. The mirror substrate is fixed on the rib structures.Hence, a state wherein the mirror structure is arranged apart above theelectrode substrate while facing the common electrode is obtained.

As described above, according to the present invention, the drivingelectrodes are arranged on the mirror structure. The electrode substrateside need only have the integrally formed common electrode. It ispossible to form a large step difference on the electrode substrate sidebecause no fine pattern formation is necessary. Hence, the pivot angleof the mirror can increase in low-voltage driving without any increasein the cost.

An example of a method of manufacturing the mirror substrate 1500 shownin FIG. 75 will be described next. First, as shown in FIG. 76A, an SOIsubstrate having, on a silicon base 1601 with a plane orientation (100),a buried insulating layer 1602 made of silicon oxide and having athickness of about 1 μm, and a 10-μm thick single-crystal silicon layer(SOI layer) 1603 is prepared. An oxide layer 1604 is formed on thesurface of the SOI layer 1603, and an oxide layer 1605 is formed on thelower surface of the silicon base 1601 by, e.g., thermal oxidation.

As shown in FIG. 76B, a metal layer 1606 is formed on the oxide layer1604. The metal layer 1606 is formed by, e.g., forming an aluminum filmby sputtering or vapor deposition. As shown in FIG. 76C, a resist masklayer 1701 with a photoresist pattern formed by known photolithographyis formed on the metal layer 1606. The metal layer 1606 is etched byusing the resist mask layer 1701 as a mask. When the resist mask layer1701 is removed, the driving electrodes 1503 a and 1503 c are formed onthe oxide layer 1604, as shown in FIG. 76D. The fabrication is done byusing well-known dry etching such as reactive ion etching. FIGS. 76C to76L, show a section and therefore do not illustrate the drivingelectrodes 1503 b and 1503 d shown in FIG. 75.

As shown in FIG. 76E, a resist mask layer 1702 with a photoresistpattern formed by known photolithography is formed on the metal layer1604 including the driving electrodes 1503 a and 1503 c. The metal layer1604 is etched by using the resist mask layer 1702 as a mask. At thistime, directional etching such as reactive ion etching is performed toexpose the surface of the SOI layer 1603 at the etched portions. Withthis process, an inorganic mask layer 1604 a with a mask pattern ofsilicon oxide is formed, as shown in FIG. 76F. At this time, a patternto form scribe lines serving as a guide in dicing is provided in aregion (not shown) of the resist mask layer 1702.

The resist mask layer 1702 is removed by ashing using ozone or oxygenplasma. As shown in FIG. 76G, the SOI layer 1603 is etched by dryetching using the inorganic mask 1604 a as a mask. With this etching,the base 1501, movable frame 1502, mirror (mirror structure) 1503,connectors (not shown), and mirror connectors (not shown) are formed.That is, the basic structure of the mirror substrate is complete. Thescribe line pattern formed in the region (not shown) of the resist masklayer 1702 is also transferred to the inorganic mask layer 1604 a andthen to the SOI layer 1603. The mirror structure may connect to the basewithout the movable frame.

A resin is applied onto the inorganic mask 1604 a including the drivingelectrodes 1503 a and 1503 c to form a resin film 1711 that fills thespaces between the patterns of the inorganic mask 1604 a and the spacesbetween the structures formed in the SOI layer 1603, as shown in FIG.76H. The resin film 1711 is etched back to form a protective layer 1712that exposes the surfaces of the driving electrodes 1503 a and 1503 cand the inorganic mask layer 1604 a and fills the spaces between thepatterns of the inorganic mask layer 1604 a and the spaces between thestructures formed in the SOI layer 1603, as shown in FIG. 76I.

The inorganic mask layer 1604 a is etched by using the drivingelectrodes 1503 a and 1503 c as a mask and removed so that the drivingelectrodes 1503 a and 1503 c exist on an insulating layer 1506 on theSOI layer 1603, as shown in FIG. 76J. Instead of selectively removingthe inorganic mask layer 1504 a, it may be used as the insulating layer1506. Next, the oxide layer 1605 and silicon base 1601 are etched byusing a mask pattern (frame formation mask pattern) formed by knownphotolithography to form the frame portion 1504, as shown in FIG. 76K.The mask pattern is removed. Then, the oxide layer 1605 and the buriedinsulating layer 1602 exposed inside the frame portion 1504 are removedby, e.g., wet etching using an alkaline solution or dry etching, asshown in FIG. 76L, so that the frame portion 1504 is fixed to the base1501 through the insulating layer 1505.

After that, for example, a reflecting film made of a metal film of,e.g., Au is formed on the surface of the mirror 1503 with the frameportion 1504 by, e.g., vapor deposition. A step of bonding the formedmirror substrate to the electrode substrate to form a mirror device, astep of packaging the mirror device and fixing it by die bonding, and astep of wire-bonding the terminals of the package to the terminals ofthe electrode substrate are executed. Then, the protective layer 1712 isremoved by, e.g., ashing using oxygen plasma to form a space between thebase 1501, movable frame 1502, and mirror 1503 to make the movable frame1502 and mirror 1503 pivotable. With the above-described bonding, themirror substrate 1500 is fixed on the rib structures 1531 provided onthe electrode substrate 1530. The protective layer 1712 may be removedbetween the above-described packaging steps.

For example, it is possible to suppress damage to the connectors evenwhen an external mechanical vibration is added in loading and fixing themirror substrate 1500 in a vapor deposition apparatus to form theabove-described reflecting film. Similarly, it is possible to suppressdamage to the connectors in the step of bonding the mirror substrate tothe electrode substrate to form the mirror device, the step of packagingthe mirror device and fixing it by die bonding, and the step ofwire-bonding the terminals of the package to the terminals of theelectrode substrate.

The above-described manufacturing method is merely an example. Anothermanufacturing method is also usable for forming the or substrate shownin FIG. 75. For example, the structures such as the mirror and movableframe are formed after formation of the driving electrodes. However, thedriving electrodes may be formed after formation of the mirror andmovable frame. The driving electrodes are formed by etching using a maskpattern as a mask. However, the present invention is not limited tothis. The driving electrodes may be formed by so-called lift-off.

Interconnections connected to the driving electrodes 1503 a to 1503 dformed on the mirror 1503 will be described next. For example, as shownin schematic plan view of FIG. 77, the driving electrodes 1503 a to 1503d can be led to the side of the base 1501 by interconnections 1803 a to1803 d which pass through the connectors 1501 a and 1501 b and mirrorconnectors 1502 a and 1502 b. The driving electrodes are symmetricalwith respect to the center of the mirror (mirror structure) 1503.

As shown in the partial enlarged perspective view of FIG. 78, aninterconnection 1813 b provided on the insulating layer 1506 formed onone surface of the mirror connector 1502 a may connect to the drivingelectrode 1503 b, and an interconnection 1813 a provided on aninsulating layer 1516 formed on the other surface may connect to thedriving electrode 1503 a. The interconnection 1813 a connects to thedriving electrode 1503 a through a plug (not shown) extending throughthe insulating layer 1506, mirror 1503, and insulating layer 1516. Thisalso applies to the driving electrodes 1503 c and 1503 d.

As shown in the partial enlarged perspective view of FIG. 79, aninterconnection 1823 b provided on the insulating layer 1506 formed onone surface of the mirror connector 1502 a may connect to the drivingelectrode 1503 b, and an interconnection 1823 a provided on aninsulating layer 1516 a on the interconnection 1823 b may connect to thedriving electrode 1503 a. This also applies to the driving electrodes1503 c and 1503 d.

As shown in the schematic plan view of FIG. 80, the mirror 1503 may havetwo divided driving electrodes 1833 a and 1833 b. The driving electrodes1833 a and 1833 b can be led to the side of the base 1501 byinterconnections 1843 a and 1843 b which pass through the connectors1501 a and 1501 b and mirror connectors 1502 a and 1502 b. The drivingelectrodes are symmetrical with respect to the center of the mirror(mirror structure) 1503.

As shown in the schematic plan view of FIG. 81, the mirror 1503 may havetwo divided driving electrodes 1853 a and 1853 b. The movable frame 1502may have two divided driving electrodes 1853 c and 1853 d. The drivingelectrodes 1853 a and 1853 b can be led to the side of the base 1501 byinterconnections 1863 a and 1863 b which pass through the connectors1501 a and 1501 b and mirror connectors 1502 a and 1502 b. The drivingelectrodes 1853 c and 1853 d can be led to the side of the base 1501 byinterconnections 1863 c and 1863 d which pass through the connectors1501 a and 1501 b.

In the above description, the common electrode 1541 is arranged in aregion to cover the projecting portion 1536. However, the presentinvention is not limited to this. It is unnecessary to arrange apatterned electrode on the side of the electrode substrate 1530. Forexample, a metal layer connected to the ground potential may be arrangedas a common electrode in the entire region of the electrode substrate1530 facing the mirror substrate 1500. This allows to omit the step ofpatterning the common electrode and more easily manufacture the mirrordevice at a low cost. The electrode substrate 1530 may have a flatcommon electrode without having the projecting portion 1536.

21st Embodiment

The 21st embodiment of the present invention will be described next.This embodiment more easily increases the interval between the mirrorsubstrate and the electrode substrate by joining the mirror substrate torib structures while inserting a gap auxiliary layer therebetween sothat the mirror substrate is spaced apart from the electrode substrateby the gap auxiliary layer and rib structures.

A structural example of a mirror device according to this embodimentwill be described with reference to FIGS. 82 and 83. FIGS. 82 and 83mainly partially illustrate a mirror device as a constituent unit of amirror array. In a mirror array, for example, mirror devices shown inFIG. 82 are arrayed two-dimensionally in a square pattern. The mirrorarray comprises a plurality of mirror substrates 1900 with mirrors and aplurality of electrode substrates 2000 with electrode portions. Themirror substrate 1900 and electrode substrate 2000 are arranged inparallel.

The mirror substrate 1900 comprises a plate-shaped base 1910,ring-shaped movable frame 1920, and disk-shaped mirror 1930. The base1910 has an opening having an almost circular shape when viewed from theupper side. The movable frame 1920 is arranged in the opening of thebase 1910 and connected to the base 1910 by a pair of connectors 1911 aand 1911 b. The movable frame 1920 also has an opening having an almostcircular shape when viewed from the upper side. The mirror 1930 isarranged in the opening of the movable frame 1920 and connected to themovable frame 1920 by a pair of mirror connectors 1921 a and 1921 b. Aframe portion 1940 surrounding the movable frame 1920 and mirror 1930 isformed at the periphery of the base 1910. The frame portion 1940 isfixed to the base 1910 through an insulating layer 1950. Additionally,in the mirror device shown in FIGS. 82 and 83, the base 1910 has, at itsperipheral portion, a gap auxiliary layer 2101 on a surface (lowersurface) facing the electrode substrate 2000. The gap auxiliary layer2101 may have a frame shape.

The connectors 1911 a and 1911 b including zigzag torsion springs andprovided in the notches of the movable frame 1920 connect the base 1910to the movable frame 1920. This structure makes the movable frame 1920connected to the base 1910 pivotable about a pivot axis (movable framepivot axis) passing through the connectors 1911 a and 1911 b. The mirrorconnectors 1921 a and 1921 b including zigzag torsion springs providedin the note the movable frame 1920 connect the movable frame 1920 to themirror 1930. This structure makes the mirror 1930 connected to themovable frame 1920 pivotable about a pivot axis (mirror pivot axis)passing through the mirror connectors 1921 a and 1921 b. The movableframe pivot axis and mirror pivot intersect each other at a right angle.

The electrode substrate 2000 has a projecting portion 2020 and ribstructures 2010 provided at the periphery of the projecting portion2020. The projecting portion 2020 includes a third terrace 2023 having atruncated pyramidal shape, a second terrace 2022 formed on the uppersurface of the third terrace 2023 and having a truncated pyramidalshape, and a first terrace 2021 formed on the upper surface of thesecond terrace 2022 and having a truncated pyramidal shape. The uppersurface of the electrode substrate 2000 including the outer surface ofthe projecting portion 2020 has sector electrode 2040 a to 2040 d formedin a circle concentric to the mirror 1930 on the counter or substrate1900. Interconnection 2070 are formed around the projecting portion 2020on the electrode substrate 2000. The electrode 2040 a to 2040 d connectto the interconnections 2070 through leads 2041 a to 2041 d. Theelectrodes may be arranged without forming the projecting portion 2020.The interconnections need not be formed on the surface of the electrodesubstrate with the electrodes and may be arranged in the electrodesubstrate by via interconnections.

The mirror 1930 on the mirror substrate 1900 faces the electrode 2040 ato 2040 d on the counter electrode substrate 2000. The gap auxiliarylayer 2101 provided on the lower surface of the base 1910 is joined tothe upper surfaces of the rib structures 2010. FIGS. 82 and 83 show astate wherein the mirror substrate 1900 is spaced apart from theelectrode substrate 2000 for the illustrative convenience. According tothe mirror device shown in FIGS. 82 and 83, the rib structures 2010 andgap auxiliary layer 2101 form the gap between the mirror substrate 1900and the electrode substrate 2000. In other words, the rib structures2010 and gap auxiliary layer 2101 join the mirror substrate 1900 to theelectrode substrate 2000.

Hence, a desired gap is formed by the rib structures 2010 and gapauxiliary layer 2101. This enables to suppress their thickness. As aresult, for example, the step difference on the mirror substrate 1900 isnot so large even if the gap auxiliary layer 2101 exists. Hence, a finepattern such as the connectors 1911 a and 1911 b and mirror connectors1921 a and 1921 b can be formed accurately. The rib structures 2010 andgap auxiliary layer 2101 support the mirror substrate 1900 and electrodesubstrate 2000 while spacing them apart from each other by apredetermined distance, thereby forming, between the mirror substrate1900 and the electrode substrate 2000, a space where a movable structuresuch as the mirror 1930 can move. It is therefore necessary to only formthe rib structures 2010 in a region on the electrode substrate 2000without the electrodes. The gap auxiliary layer 2101 need only bearranged in accordance with the positions of the thus formed ribstructures 2010.

To increase the difference of elevation of the projecting portion, theinterval (gap) between the mirror substrate and the electrode substrateis increased. Conventionally, it is difficult to increase the gap. In aprior art, for example, convex portions (rib structures) 8360 a and 8360b formed by fabricating an electrode substrate 8301 form a gap between amirror substrate 8201 and the electrode substrate 8301, as shown in thesectional view of FIG. 13. Alternatively, a frame portion 8241 of amirror substrate 8200 forms a gap between the mirror substrate 8200 andan electrode substrate 8300, as shown in the sectional view of FIG. 86.Support portions 8260 formed on a frame portion (base) 8210 of themirror array 800 may form a gap between the mirror substrate 8200 andthe electrode substrate 8300, as shown in the sectional view of FIG. 87.

In the structure shown in FIG. 13, however, the electrodes andinterconnections are formed after the convex portions 8360 a and 8360 bare formed. Hence, the convex portions 8360 a and 8360 b cannot be sohigh. Generally, electrodes and interconnections are formed byphotolithography. Photoresist application and exposure are verydifficult in a region with a large step difference. For example innormal photolithography, the step difference that allows patternformation is about 70 μm.

In the structure shown in FIG. 86, the frame portion 8241 of an SOIsubstrate can be formed by fabricating a thick silicon base. However,since the gap is about 100 to 200 μm, the base is thinned to about 100to 200 μm. However, such a thin mirror substrate has a remarkably lowstrength and readily breaks in handling in the mirror formation step ormounting step. In the structure shown in FIG. 87, the support portions8260 produces a large step difference on the SOI layer with the mirror8230 and frame portion 8210. If a large step difference exists, it isvery difficult to pattern a fine structure such as connectors.

A mirror element according to this embodiment has the above-describedstructure. Since the mirror substrate is joined to the rib structuresvia the gap auxiliary layer, the gap auxiliary layer and rib structuresspace the mirror substrate apart from the electrode substrate so thatthe interval between the mirror substrate and the electrode substratecan more easily increase. The electrode substrate may have a projectingportion formed into an almost conical shape from the base and facing themirror. Electrodes may be formed on the projecting portion.

An example of a method of manufacturing the mirror substrate 1900included in the mirror device of this embodiment will be described next.First, as shown in FIG. 84A, an SOI substrate having, on a silicon base1940 a with a plane orientation (100), the insulating layer 1950 made ofsilicon oxide and having a thickness of, e.g., about 1 μm, and a 10-μmthick single-crystal silicon layer (SOI layer) 1901 is prepared. Theinsulating layer 1950 is a buried insulating layer. As shown in FIG.84B, the gap auxiliary layer 2101 is formed on the silicon base 1940 a.The gap auxiliary layer 2101 can be formed by, e.g., selectively forminga seed layer in the formation region of the gap auxiliary layer 2101 andforming a metal layer on the seed layer by electrolytic plating. The gapauxiliary layer 2101 needs to be formed at the boundary of a regioncorresponding to one mirror device.

As shown in FIG. 84C, a mask pattern 2102 is formed on the SOI layer1901. The SOI layer 1901 is etched by using the mask pattern 2102 as amask. At this time, directional etching such as reactive ion etching isperformed to expose the surface of the insulating layer 1950 at theetched portions. With this etching, the base 1910, movable frame 1920,mirror (mirror structure) 1930, connectors (not shown), and mirrorconnectors (not shown) are formed. That is, the basic structure of themirror substrate is complete, as shown in FIG. 84D.

After the mask pattern 2102 is removed, a mask pattern (not shown) isformed on the lower surface of the silicon base 1940 a. The mask patterncorresponds to one mirror portion of a mirror array and has a squareopening region for each mirror. The silicon base 1940 a is etched by dryetching by using a CF-based gas and the mask pattern as a mask until theinsulating layer 1950 exposes. This process may be wet etching.

After that, the mask pattern is removed. The mask pattern can be removedby, e.g., ashing or appropriate etching. The insulating layer 1950exposed inside the formed opening region is removed to form the frameportion 1940, as shown in FIG. 84E. In FIGS. 84A to 84E, the connectorsand mirror connectors are formed after formation of the gap auxiliarylayer. However, the present invention is not limited to this. The gapauxiliary layer may be formed after the precise fine patterns of theconnectors and mirror connectors are formed.

Another example of the method of manufacturing the mirror substrate 1900included in the mirror device of this embodiment will be described next.

First, as shown in FIG. 85A, an SOI substrate having, on the siliconbase 1940 a with a plane orientation (100), the insulating layer 1950made of silicon oxide and having a thickness of, e.g., about 1 μm, andthe 10-μm thick single-crystal silicon layer (SOI layer) 1901 isprepared. This is the same as in the manufacturing method described withreference to FIGS. 84A to 84E.

As shown in FIG. 85B, a mask pattern 2111 is formed on the SOI layer1901. The SOI layer 1901 is etched by using the mask pattern 2102 as amask. At this time, directional etching such as reactive ion etching isperformed to expose the surface of the insulating layer 1950 at theetched portions. With this etching, the base 1910, movable frame 1920,mirror or structure) 1930, connectors (not shown), and mirror connectors(not shown) are formed. That is, the basic structure of the mirrorsubstrate is complete, as shown in FIG. 85C.

After the mask pattern 2111 is removed, a gap auxiliary layer 2101 a of,e.g. glass is formed on the base 1910 (SOI layer 1901), as shown in FIG.85D. The gap auxiliary layer 2101 a of glass is joined to the siliconbase 1910 by, e.g., known anodic bonding.

A mask pattern is formed on the lower surface of the silicon base 1940a. The mask pattern corresponds to one mirror portion of a mirror arrayand has a square opening region for each mirror. The silicon base 1940 ais etched by dry etching by using a CF-based gas and the mask pattern asa mask until the insulating layer 1950 exposes. This process may be wetetching.

After that, the mask pattern is removed. The mask pattern can be removedby, e.g., ashing or appropriate etching. The insulating layer 1950exposed inside the formed opening region is removed to form the frameportion 1940, as shown in FIG. 85E.

22nd Embodiment

The 22nd embodiment of the present invention will be described next.

The constituent elements of a conventional mirror device 8200 shown inFIGS. 11 to 13 and, particularly, the constituent elements (to bereferred to as movable members hereinafter) of a mirror substrate 8201including a movable frame 8220, mirror 8230, movable frame connectors8211 a and 8211 b, and mirror connectors 8221 a and 8221 b are spacedpart from adjacent members to make the mirror 8230 pivot about themirror pivot axis and movable frame pivot axis. For this reason,external impact on the mirror device 8200 may cause the movable membersof the mirror substrate 8201 to collide against adjacent members andbreak. Hence, the movable members of the mirror substrate 8201 and themembers adjacent to the movable members must have intervals to preventcollusion between adjacent constituent elements in case of externalimpact.

This embodiment has been made to solve the above-described problem andhas as its object to provide a mirror device and a mirror array whichhave a high impact resistance.

This embodiment will be described below in detail with reference to theaccompanying drawings. A mirror array according to this embodiment setsthe intervals between the movable members of the mirror substrate of amirror device included in the mirror array and members adjacent to themovable members to a predetermined value. The same names and referencenumerals as in the mirror device shown in FIGS. 14 to 16B denote thesame constituent elements in FIG. 88 that shows this embodiment, and adescription thereof will be omitted as needed.

A mirror device included in a mirror array according to this embodimentsets four intervals d1 to d4 in a mirror substrate 200 to predeterminedvalues.

As well shown in an enlarged view a of FIG. 88, the interval d1 is theinterval between the edge of an opening 210 a of a frame portion 210 andthe arc-shaped edge of a movable frame 220.

As well shown in an enlarged view b of FIG. 88, the interval d2 is theinterval between a mirror connector 221 b and a second notch 223 b and,more specifically, the interval between an end of the mirror connector221 b in a direction perpendicular to the mirror pivot axis and an edgeof the second notch 223 b adjacent to the end and parallel to the mirrorpivot axis. The interval d2 also includes the interval between an end ofthe mirror connector 221 b in a direction parallel to the mirror pivotaxis and an edge of the second notch 223 b adjacent to the end andperpendicular to the mirror pivot axis.

The interval d2 also includes the interval between a mirror connector221 a and a second notch 223 a, the interval between a movable frameconnector 211 a and a first notch 222 a, and the interval between amovable frame connector 211 b and a first notch 222 b. Their detailedcontents are as follows.

The interval between the mirror connector 221 a and the second notch 223a indicates the interval between an end of the mirror connector 221 a ina direction perpendicular to the mirror pivot axis and an edge of thesecond notch 223 a adjacent to the end and parallel to the mirror pivotaxis. This interval also includes the interval between an end of themirror connector 221 a in a direction parallel to the mirror pivot axisand an edge of the second notch 223 a adjacent to the end andperpendicular to the mirror pivot axis.

The interval between the movable frame connector 211 a and the firstnotch 222 a indicates the interval between an end of the movable frameconnector 211 a in a direction perpendicular to the movable frame pivotaxis and an edge of the first notch 222 a adjacent to the end andparallel to the movable frame pivot axis. This interval also includesthe interval between an end of the movable frame connector 211 a in adirection parallel to the movable frame pivot axis and an edge of thefirst notch 222 a adjacent to the end and perpendicular to the movableframe pivot axis.

The interval between the movable frame connector 211 b and the firstnotch 222 b indicates the interval between an end of the movable frameconnector 211 b in a direction perpendicular to the movable frame pivotaxis and an edge of the first notch 222 b adjacent to the end andparallel to the movable frame pivot axis. This interval also includesthe interval between an end of the movable frame connector 211 b in adirection parallel to the movable frame pivot axis and an edge of thefirst notch 222 b adjacent to the end and perpendicular to the movableframe pivot axis.

As well shown in an enlarged view c of FIG. 88, the interval d3 is theinterval between the edge of an opening 220 a of the movable frame 220and the edge of a mirror 230.

As well shown in an enlarged view d of FIG. 88, the interval d4 is theinterval between the mirror connector 221 a and the movable frame 220and, more specifically, the interval between an end of the mirrorconnector 221 a on the side connected to the mirror 230 and formed alongthe mirror pivot axis and an edge of a communicating portion 224 a thatcommunicates the second notch 223 a of the movable frame 220 to theopening 220 a.

The interval d4 also includes the interval between the movable frame 220and the mirror connector 221 b as the counterpart of the mirrorconnector 221 a and, more specifically, the interval between an end ofthe mirror connector 221 b on the side connected to the mirror 230 andformed along the mirror pivot axis and an edge of a communicatingportion 224 b that communicates the second notch 223 b of the movableframe 220 to an opening 220 b.

The intervals d1 to d4 are set in the following way.

Let m [kg] be the mass of a movable portion of the mirror device, and G[m/s²] be the acceleration applied to the movable portion. The movableportion receives a force given by mG. Let d be the displacement amountof the movable portion, and k be the spring constant of the movableframe connectors 211 a and 211 b and mirror connectors 221 a and 221 bthat support the movable portion. The relationship between m, G, k, andd is given bymG=kd  (15)

From equation (15), the displacement amount d is calculated byd=mG/k  *16)

The intervals d1 to d4 are set to be larger than the value of thedisplacement amount d of equation (16). This prevents the movable memberof the mirror substrate 200 from contacting a member adjacent to themovable member and breaking even when the acceleration G is applied tothe mirror array according to this embodiment.

The value of the acceleration G can freely be set as needed on the basisof the desired value of impact resistance to be imparted to the mirrorarray of this embodiment.

The movable portion of the mirror device indicates the movable frame 220and mirror 230. The movable frame 220 has opening portions such as theopening 220 a, first notches 222 a and 222 b, and second notches 223 aand 223 b. To more reliably prevent the mirror device from breaking, themass is set assuming that the movable portion is a perfect circlewithout openings. Let r be the radius of the movable frame 220, ρ be thedensity of the material of the movable frame 220 and mirror 230, and hbe the thickness of the movable frame 220 and mirror 230. At this time,the mass in of the movable portion is given by πr²ρh.

The mirror connectors 221 a and 221 b support the mirror 230 anddisplace in accordance with the acceleration applied to the mirror 230.The intervals related to the mirror connectors 221 a and 221 b, i.e.,the intervals d2 related to the mirror connectors 221 a and 221 b andthe intervals d3 and d4 are set to be larger than the displacement dcalculated from equation (16) by substituting the mass of the mirror 230into the mass m. In this embodiment, to more reliably prevent the mirrordevice from breaking, the mass m of equation (16) uses a valuecalculated from πr²ρh by using the radius r of the movable frame 220, asdescribed above.

The zigzag torsion springs included in the movable frame connectors 211a and 211 b and mirror connectors 221 a and 221 b displace in the X andY directions, as shown in FIG. 89. For the spring constants k of themovable frame connectors 211 a and 211 b and mirror connectors 221 a and221 b, a spring constant k_(x) in the X-axis and a spring constant k_(y)in the Y direction must be taken into consideration. The mirror array ofthis embodiment employs the smallest one of the spring constants k_(x)and k_(y), which is substituted into equation (16). This more reliablyprevents the movable member of the mirror substrate 200 from contactinga member adjacent to the movable member and breaking even when theacceleration G is applied to the mirror device. Hence, the mirror arrayaccording to this embodiment has high impact resistance.

In this embodiment, the pair of movable frame connectors 211 a and 211 bsupport the movable frame 230. The pair of mirror connectors 221 a and221 b support the mirror 230. The spring constant k of equations (15)and (16) indicates the value of the spring constant of the pair ofmovable frame connectors 211 a and 211 b or the spring constant of thepair of mirror connectors 221 a and 221 b.

The spring constants of the movable frame connectors 211 a and 211 b andmirror connectors 221 a and 221 b may be calculated after setting theintervals d1 to d4 in advance. In this case, together with the mass m ofthe movable portion of the mirror device, the acceleration G applied tothe movable portion, the preset values of the intervals d1 to d4 aresubstituted into the displacement amount d of equation (17) rewrittenfrom equation (15), thereby calculating the spring constant k. On thebasis of the calculated spring constant k, the shapes and sizes of themovable frame connectors 211 a and 211 b and mirror connectors 221 a and221 b are set such that the spring constants k_(x) and k_(y) of themovable frame connectors 211 a and 211 b and mirror connectors 221 a and221 b in the respective directions exceed the spring constant kcalculated from equation (17). This also prevents the movable member ofthe mirror substrate 200 from contacting a member adjacent to themovable member.k=mG/d  (17)

A detailed setting example of the spring constant will be describednext.

For example, assume that the mass of the movable portion of the mirrordevice is 1.6×10⁻¹ [kg], the acceleration applied to the mirror deviceis 100 G, and the intervals d1 to d4 are 10 [μm]. The spring constant kis calculated by(1.6×10⁻⁸)×(100×9.81)/10×10⁻⁶≈1.57

Hence, setting the shapes and the like of the movable frame connectors211 a and 211 b and mirror connectors 221 a and 221 b such that thespring constants of the movable frame connectors 211 a and 211 b andmirror connectors 221 a and 221 b exceed 1.57 allows to prevent themovable member of the mirror substrate 200 from contacting a memberadjacent to the movable member.

A method of manufacturing a mirror array according to this embodimentwill be described next.

The mirror substrate 200 is formed from an SOI (Silicon On Insulator)substrate.

First, a side (major surface: SOI layer) of the SOI substrate with aburied insulating layer 250 undergoes known photolithography and etchingsuch as DEEP RIE to form, in the single-crystal silicon layer, trenchesconforming to the shapes of the frame portion 210, movable frameconnectors 211 a and 211 b, movable frame 220, mirror connectors 221 aand 221 b, and mirror 230.

At this time the trenches are formed such that the above-describedintervals d1 to d4 equal or exceed the displacement amount d calculatedon the basis of equation (16). More specifically, the mass m of themovable member including the movable frame 220 and mirror 230 iscalculated from, e.g., the shapes of the trenches. The spring constantsk of the movable frame connectors 211 a and 211 b and mirror connectors221 a and 221 b are calculated from, e.g., the shapes of the trenches.The acceleration G that the mirror array should stand is set.Substituting these values into equation (16) yields the displacementamount d, and the trenches are formed such that the intervals d1 to d4equal or exceed the displacement amount d.

A resist pattern with openings in predetermined regions corresponding tothe trenches is formed on the lower surface of the SOI substrate. Thesilicon is selectively etched from the lower surface of the SOIsubstrate by dry etching using, e.g., SF₆. In this etching, the openingand frame-shaped member 240 are formed on the lower surface of the SOIsubstrate by using the buried insulating layer 250 as an etching stopperlayer. The silicon may be etched by wet etching using, e.g., potassiumhydroxide.

A region of the buried insulating layer 250 exposed to the opening isremoved by dry etching using, e.g., CF₄ gas. The buried insulating layer250 may be removed by using hydrofluoric acid.

On the other hand, an electrode substrate 300 is formed from, e.g., asilicon substrate.

First, a silicon substrate is selectively etched by using, as a mask, apredetermined mask pattern made of a silicon nitride film or siliconoxide film and a potassium hydroxide solution. A base 310, first tothird terraces 321 to 323, pivot 330, outer trench 350, and convexportions 360 a and 360 b are formed by repeating the above-describedprocess.

The surface of the silicon substrate on the etched side is oxidized toform a silicon oxide film.

A metal film is formed on the silicon oxide film by, e.g., vapordeposition and patterned by known photolithography and etching to formelectrodes 340 a to 340 d, leads 341 a to 341 d, and interconnections370.

With this process, the electrode substrate 300 having theabove-described shape is formed.

Then, the mirror substrate 200 is bonded to the electrode substrate 300to form a mirror array having a mirror device that moves the mirror 230by applying an electric field to the electrodes 340 a to 340 d.

The thus manufactured mirror array prevents the movable member of themirror substrate 200 from contacting another constituent element of themirror substrate 200 adjacent to the movable member and the mirrordevice from breaking even when the acceleration C is applied. Hence, themirror array according to this embodiment has high impact resistance.

In this embodiment, the movable frame connectors 211 a and 211 b areprovided in the first notches 222 a and 222 b formed in the movableframe 220. The movable frame connectors 211 a and 211 b may be providedin notches formed in the frame portion 210. The intervals between themovable member and a member adjacent to the movable member in this casewill be described with reference to FIG. 90. The same names andreference numerals as in the mirror substrate 200 shown in FIG. 88denote the same constituent elements in FIG. 90, and a descriptionthereof will be omitted as needed.

In the mirror substrate 200 shown in FIG. 90, the pair of movable frameconnectors 211 a and 211 b including zigzag torsion springs and providedin a pair of third notches 212 a and 212 b of the frame portion 210connect the frame portion 210 to the movable frame 220. This structuremakes the movable frame 220 pivotable about a pivot axis (movable framepivot axis) passing through the pair of movable frame connectors 211 aand 211 b.

In the mirror array shown in FIG. 90, five intervals d1 to d5 in themirror substrate 200 are set to be larger than the value calculated byequation (16).

The intervals d1 to d4 are the same as in the mirror substrate 200 shownin FIG. 88, and a description thereof will be omitted as needed.

As well shown in an enlarged view b of FIG. 90, the interval d2 is theinterval between the mirror connector 221 b and the second notch 223 band, more specifically, the interval between an end of the mirrorconnector 221 b in a direction perpendicular to the mirror pivot axisand an edge of the second notch 223 b adjacent to the end and parallelto the mirror pivot axis. The interval d2 also includes the intervalbetween an end of the mirror connector 221 b in a direction parallel tothe mirror pivot axis and an edge of the second notch 223 b adjacent tothe end and perpendicular to the mirror pivot axis.

The interval d2 also includes the interval between the mirror connector221 a and the second notch 223 a, the interval between the movable frameconnector 211 a and the third notch 212 a, and the interval between themovable frame connector 211 b and the third notch 212 b. Their detailedcontents are as follows.

The interval between the mirror connector 221 a and the second notch 223a indicates the interval between an end of the mirror connector 221 a ina direction perpendicular to the mirror pivot axis and an edge of thesecond notch 223 a adjacent to the end and parallel to the mirror pivotaxis. This interval also includes the interval between an end of themirror connector 221 a in a direction parallel to the mirror pivot axisand an edge of the second notch 223 a adjacent to the end andperpendicular to the mirror pivot axis.

The interval between the movable frame connector 211 a and the thirdnotch 212 a indicates the interval between an end of the movable frameconnector 211 a in a direction perpendicular to the movable frame pivotaxis and an edge of the third notch 212 a adjacent to the end andparallel to the movable frame pivot axis. This interval also includesthe interval between an end of the movable frame connector 211 a in adirection parallel to the movable frame pivot axis and an edge of thethird notch 212 a adjacent to the end and perpendicular to the movableframe pivot axis.

The interval between the movable frame connector 211 b and the thirdnotch 212 b indicates the interval between an end of the movable frameconnector 211 b in a direction perpendicular to the movable frame pivotaxis and an edge of the third notch 212 b adjacent to the end andparallel to the movable frame pivot axis. This interval also includesthe interval between an end of the movable frame connector 211 b in adirection parallel to the movable frame pivot axis and an edge of thethird notch 212 b adjacent to the end and perpendicular to the movableframe pivot axis.

As well shown in an enlarged view e of FIG. 90, the interval d5 is theinterval between the movable frame connector 211 a and the frame portion210 and, more specifically, the interval between an end of the movableframe connector 211 a on the side connected to the movable frame 220 andformed along the movable frame pivot axis and an edge of a communicatingportion 213 a that communicates the third notch 212 a of the frameportion 210 to the opening 210 a.

The interval d5 also includes the interval between the movable frameconnector 211 b as the counterpart of the movable frame connector 211 aand the frame portion 210 and, more specifically, the interval betweenan end of the movable frame connector 211 b on the side connected to themovable frame 220 and formed along the movable frame pivot axis and anedge of a communicating portion 213 b that communicates the third notch212 b of the frame portion 210 to the opening 210 a.

Even in this mirror array, the intervals d1 to d5 are set to be largerthan the value of the displacement amount d of equation (16). Thisprevents the movable member of the mirror substrate 200 from contactinga member adjacent to the movable member and breaking even when theacceleration G is applied.

As described above, according to this embodiment, letting m be the massof the mirror, G be the acceleration applied to the mirror device ormirror array, and k be the spring constant of an elastic member, theinterval between the mirror and a frame member supporting the mirror isset to mG/k or more. This prevents the mirror from colliding against theframe member and breaking even when the acceleration G is applied to themirror device or mirror array. Hence, the mirror device and mirror arrayof the present invention have high impact resistance.

23rd Embodiment

The 23rd embodiment of the present invention will be described below.

Biaxial rotation 3D-MEMS (Micro Electro Mechanical Systems) opticalswitches have received a great deal of attention as a hardwaretechnology to implement large-scale optical switches. FIG. 91 shows aconventional optical switch. Referring to FIG. 91, reference numeral8511 a denotes an input optical fiber array; 8511 b, an output opticalfiber array; 8512 a, an input-side collimator array; 8512 b, anoutput-side collimator array; 8513 a, an input-side mirror array; and8513 b, an output-side mirror array. Each of the optical fiber arrays8511 a and 8511 b includes a plurality of optical fibers arrayedtwo-dimensionally. Each of the collimator arrays 8512 a and 8512 bincludes a plurality of microlenses arrayed two-dimensionally. Each ofthe mirror arrays 8513 a and 8513 b includes a plurality of mirrordevices 8514 a and 8514 b arrayed two-dimensionally. The arrow in FIG.91 indicates the traveling direction of a light beam.

An optical signal that has exited an input port of the input opticalfiber array 8511 a is converted into a light beam by a microlens of theinput-side collimator array 8512 a, sequentially reflected by theinput-side mirror array 8513 a and output-side mirror array 8513 b,condensed by a microlens of the output-side collimator array 8512 b, andguided to an optical fiber of the output optical fiber array 8511 b. Themirror of each of the mirror devices 8514 a and 8514 b included in themirror arrays 8513 a and 8513 b can pivot about two axes to reflectincident light in a desired direction corresponding to the tilt angle ofthe mirror. It is possible to connect an arbitrary input optical fiberto an arbitrary output optical fiber and switch the optical path byappropriately controlling the tilt angles of mirrors of the input-sidemirror array 8513 a and output-side mirror array 8513 b.

The most characteristic components of the above-described optical switchare the mirror arrays 8513 a and 8513 b. The mirror devices 8514 a and8514 b included in the mirror arrays 8513 a and 8513 b have thestructure shown in FIGS. 107 and 108.

In the mirror device shown in FIGS. 107 and 108, a tilt angle θ of amirror 8103 with respect to the moment by an electrostatic force has nodirection dependency. The mirror 8103 exhibits the same behaviorregardless of whether it pivots about the X-axis or Y-axis if torsionsprings 8104 and 8105 have the same structure, and the moment by theelectrostatic force is same. Even in pivotal movement in an arbitrarydirection between the X-axis and the Y-axis, the mirror 8103 exhibitsthe same tilt angle θ in the same moment.

However, driving electrodes 8003-1 to 8003-4 arranged facing the mirror8103 to give an electrostatic force to control the biaxial pivotalmovement of the mirror 8103 are normally formed as four dividedelectrodes. In addition, the power supply voltage normal has an upperlimit, and the tilt angle θ of the mirror 8103 has a directiondependency for these reasons. Since the driving voltage applied to thefour driving electrodes 8003-1 to 8003-4 controls the pivotal movementof the mirror 8103, the anisotropy of the tilt angle θ of the mirror8103 poses no problem in a range where the tilt angle θ is small. Whenthe power supply voltage is limited, and the mirror 8103 can pivot up toonly a certain finite tilt angle θ, the tilt angle θ of the mirror 8103is large when it pivots in directions (the directions of the X- andY-axes in FIG. 107) parallel to the parting lines of the four divideddriving electrodes 8003-1 to 8003-4 and small when the mirror pivots indirections 45° with respect to the parting lines. This is because twodriving electrodes can attract the mirror 8103 by the electrostaticforce in the directions parallel, to the parting lines of the drivingelectrodes 8003-1 to 8003-4 while only one driving electrode needattract the mirror 8103 in the directions 45° with respect to theparting lines.

Hence, when the mirror arrays 8513 a and 8513 b face each other, as inFIG. 91, a region (region that receives reflected light from the mirror8103) 301 on the output-side mirror array 8513 b, which is scannable bythe mirrors 8103 of the input-side mirror devices 8514 a arranged in themirror array 8513 a, narrows not in the directions parallel to theparting lines of the driving electrodes 8003-1 to 8003-4 of theinput-side mirror devices 8514 a but in the directions 45° with respectto the parting lines, as shown in FIG. 92. This also applies to a regionon the input-side mirror array 8513 a, which is scannable by the mirrors8103 of the output-side mirror devices 8514 b arranged in the mirrorarray 8513 b.

To the contrary, when the mirrors 8103 exit in a rectangular region 8602of the output-side mirror array 8513 b, the mirrors 8103 arrangedoutside a scannable region 8601 must also effectively function. Hence, ahigher driving voltage must be applied to the driving electrodes 8003-1to 8003-4 of the input-side mirror devices 8514 a to largely tilt themirrors 8103 of the input-side mirror devices 8514 a. To increase thedriving voltage for mirror control, it is necessary to change thespecifications of the power supply and also increase the insulatingproperties of interconnections and the breakdown voltage of theconnector and cable. This increases the technical difficulty, resultingin a high cost.

On the other hand, when the mirrors 8103 exit in a rectangular region303 of the output-side mirror array 8513 b, the mirrors 8103 of theinput-side mirror devices 8514 a need not tilt largely. Hence, thedriving voltage can be low. However, the scannable region 8601 is notused effectively because no mirror 8103 is arranged in the hatched partof the scannable region 8601 shown in FIG. 92. Since the optical switchrequires to have multiple channels, the number of arranged mirrors 8103is preferably increased as much as possible by effectively using thescannable region 8601.

If the driving electrode is divided into eight or 16 parts instead offour parts, and a voltage is applied to driving electrodes correspondingto ½ the total driving electrode area independently of the pivotdirection of the mirror 8103, the tilt angle θ of the mirror 8103 has noanisotropy. However, in the mirror arrays 8513 a and 8513 b, several tento several hundred mirrors 8103 must simultaneously be controlled. Thisproduces a strong demand for decreasing the number of driving electrodesprepared for one mirror 8103 to a minimum necessary number. The mirrorarrays 8513 a and 8513 b each having 100 mirrors 8103 require at least400 interconnections for four divided driving electrodes. The process ofthese interconnections is not easy. When the number of divided drivingelectrodes further increases, the number of interconnections reaches animpractical level, making the manufacture very difficult.

As described above, in the conventional optical switch, the drivingvoltage applied to the driving electrodes 8003-1 to 8003-4 of the mirrordevices 8514 a and 8514 b must be high because of the anisotropy of thetilt angle θ of the mirrors 8103 of the mirror devices 8514 a and 8514 barranged in the mirror arrays 8513 a and 8513 b. If the driving voltageis to be decreased by reducing the mirror arrangement region, themirrors cannot be arranged efficiently. In addition, if the drivingvoltage is to be decreased by increasing the number of divisions of adriving electrode, the device is hard to manufacture.

This embodiment has been made to solve the above-described problem andhas as its object to efficiently arrange mirrors in a pair of mirrorarrays provided in an optical switch while preventing any increase inthe driving voltage and the difficulty of manufacture.

An optical switch according to this embodiment will be described nextwith reference to FIG. 93. Referring to FIG. 93, reference numeral 2201d denotes an input optical fiber array; 2201 b, an output optical fiberarray; 2202 a, an input-side collimator array; 2202 b, an output-sidecollimator array; 2203 a, an input-side mirror array; and 2203 b, anoutput-side mirror array. Each of the optical fiber arrays 2201 a and2201 b includes a plurality of optical fibers arrayed two-dimensionally.Each of the collimator arrays 2202 a and 2202 b includes a plurality ofmicrolenses arrayed two-dimensionally. Each of the mirror arrays 2203 aand 2203 b includes a plurality of mirror devices 2214 a and 2214 barrayed two-dimensionally. The arrow in FIG. 93 indicates the travelingdirection of a light beam.

The optical fibers of the input optical fiber array 2201 a and themicrolenses of the input-side collimator array 2202 a are arranged in amatrix in a rectangular region 2204 a. The mirror devices 2214 a arearranged in a matrix in a rectangular region 2205 a of the input-sidemirror array 2203 a corresponding to the region 2204 a. Similarly, theoptical fibers of the output optical fiber array 2201 b and themicrolenses of the output-side collimator array 2202 b are arranged in amatrix in a rectangular region 2204 b. The mirror devices 2214 b arearranged in a matrix in a rectangular region 2205 b of the output-sidemirror array 2203 b corresponding to the region 2204 b.

As before, an optical signal that has exited an input port of the inputoptical fiber array 2201 a is converted into a light beam by a microlensof the input-side collimator array 2202 a, sequentially reflected by theinput-side mirror array 2203 a and output-side mirror array 2203 b,condensed by a microlens of the output-side collimator array 2202 b, andguided to an optical fiber of the output optical fiber array 2201 b.

FIG. 94 shows the structure of the mirror devices 2214 a and 2214 baccording to this embodiment. The same names and reference numerals asin the mirror device shown in FIG. 1 denote the same constituentelements in FIG. 94, and a description thereof will be omitted asneeded.

In the conventional optical switch shown in FIG. 91, a pair of oppositesides of a rectangular region 8515 a of the mirror array 8513 a wherethe mirror devices 8514 a are arranged are parallel to the first partinglines (e.g., X-axis direction) of the driving electrodes 8003-1 to8003-4 of the mirror devices 8514 b facing the mirror devices 8514 a. Inaddition, the other pair of opposite sides of the rectangular region8515 a are parallel to the second parting lines (e.g., Y-axis direction)of the driving electrodes 8003-1 to 8003-4 of the mirror devices 8514 b.Similarly, a pair of opposite sides of a rectangular region 8515 b ofthe mirror array 8513 b where the mirror devices 8514 b are arranged areparallel to the first parting lines of the driving electrodes 8003-1 to8003-4 of the mirror devices 8514 a. In addition, the other pair ofopposite sides of the rectangular region 8515 b are parallel to thesecond parting lines of the driving electrodes 8003-1 to 8003-4 of themirror devices 8514 a.

In this embodiment, a pair of opposite sides (e.g., 2206 and 2207 inFIG. 95) of the rectangular region 2205 b of the mirror array 2203 bwhere the mirror devices 2214 b are arranged intersect the first partinglines (e.g., X-axis direction) of driving electrodes 103-1 to 103-4 ofthe mirror devices 2214 a facing the mirror devices 2214 b at an angleof 45°. In addition, the other pair of opposite sides (e.g., 2208 and2209 in FIG. 95) of the rectangular region 2205 b intersect the secondparting lines (e.g., Y-axis direction) of the driving electrodes 103-1to 103-4 of the mirror devices 2214 a at an angle of 45°.

Similarly, a pair of opposite sides of the rectangular region 2205 a ofthe mirror array 2203 a where the mirror devices 2214 a are arrangedintersect the first parting lines of the driving electrodes 103-1 to103-4 of the mirror devices 2214 b facing the mirror devices 2214 a atan angle of 45°. In addition, the other pair of opposite sides of therectangular region 2205 a intersect the second parting lines of thedriving electrodes 103-1 to 103-4 of the mirror devices 2214 b at anangle of 45°.

The mirror arrangement method of this embodiment indicates that thearrangement regions 2205 a and 2205 a of the mirror devices 2214 a and2214 b are wide in the directions parallel to the X- and Y-axes andnarrow in the direction 45° with respect to the X- and Y-axes. That is,in this embodiment, the arrangement regions 2205 a and 2205 b of themirror devices 2214 a and 2214 b have the same area as a region 8603 inFIG. 92 in the direction 45° with respect to the X- and Y-axes andextend to the hatched part of the region 8601 in the directions parallelto the X- and Y-axes. The region 8601 is the scannable region of mirrors153 of the mirror devices 2214 a and 2214 b. Hence, it is unnecessary toincrease the maximum voltage applied to the driving electrodes 103-1 to103-4.

In this embodiment, the arrangement of mirror 203 (mirror devices 2214 aand 2214 b) of the mirror arrays 2203 a and 2203 b is optimized inconsideration of the anisotropy of the tilt angle θ of the mirror 153.This allows to effectively use the scannable region of the mirror 153and obtain an efficient mirror arrangement without increasing thedriving voltage. In this embodiment, no multiple divided drivingelectrodes such as eight or 16 divided driving electrodes are used.Since it is only necessary to change the directions of the parting linesof the driving electrodes 103-1 to 103-4, the mirror arrays 2203 a and2203 b can be manufactured easily as before.

As described above, according to this embodiment, the mirror arrangementregion of the second mirror array is set in accordance with theanisotropy of the tilt angle of the first mirror decided by thedirection of the parting line of the first driving electrode of thefirst mirror array, and the mirror arrangement region of the firstmirror array is set in accordance with the anisotropy of the tilt angleof the second mirror decided by the direction of the parting line of thesecond driving electrode of the second mirror array. This allows toeffectively use the scannable region of the first and second mirrors andobtain an efficient mirror arrangement without increasing the drivingvoltage applied to the first and second driving electrodes. In thepresent invention, no multiple divided driving electrodes such as eightor 16 divided driving electrodes are used. Hence, the mirror array canbe manufactured easily as before.

24th Embodiment

In the 23rd embodiment, the arrangement of the mirrors 153 of the mirrorarrays 2203 a and 2203 b is optimized in consideration of the anisotropyof the tilt angle θ of the mirror 153. As is known, driving electrodes103-1 to 103-4 with a staircase shape as shown in FIG. 96 can reduce thedriving voltage. This is because the distance between the drivingelectrodes 103-1 to 103-4 and the mirror 153 can be shortened withoutsacrificing the tilt angle θ of the mirror 153. In the mirror deviceshown in FIG. 96, a terrace-shaped projecting portion 120 is formed atthe center of a tower substrate 101. The driving electrodes 103-1 to103-4 are provided on the projecting portion 120.

Ideally, the conical projecting portion 120 with its apex at the centerof the mirror 153 is formed, as shown in FIG. 97. Forming the drivingelectrodes 103-1 to 103-4 on the slanting surface of the projectingportion L20 allows to attain a large pull-in angle and low-voltagedriving. At the tilt angle θ equal to or more than the pull-in angle, itis impossible to statically stably control the mirror 153. If the tiltangle θ is equal to or larger than the pull-in angle, the electrostaticforce surpasses the restoring force of torsion springs so that themirror 153 approaches the driving electrodes 103-1 to 103-4. When thedriving electrodes 103-1 to 103-4 are formed on the conical projectingportion 120, the pull-in angle is about ⅓ the angle made by the mirror153 and the slanting surface of the projecting portion 120 withoutvoltage application.

It is difficult to form such a conical structure. Actually, theterrace-shaped projecting portion approximate to the conical shape isformed by anisotropic etching of silicon, and the driving electrodes103-1 to 103-4 are formed on the terrace-shaped projecting portion 120.Anisotropic etching uses the fact that the etching rate of silicon withrespect to KOH excessively changes depending on the crystal orientation.For this reason, the (100) plane of silicon exposes in the etchingdirection while the (111) plane forms the slant of the projectingportion 120. The tilt angle of the slant is 57.4°. To approximatelyobtain an arbitrary slant angle, anisotropic etching is executed inseveral steps to form the terrace-shaped projecting portion 120 close tothe desired tilt angle.

The shape of the upper surface of each terrace of the projecting portion120 is almost square when viewed from the upper side, as indicated by106 and 107 in FIG. 98. Since upper surfaces 121 and 122 are almostsquare, the angle made by the mirror 153 and the slant changes dependingon the direction. More specifically, the angle made by the mirror 153and the slant of the projecting portion 120 is large in a direction(e.g., the Z-axis direction in FIG. 98) perpendicular to a pair ofopposite sides of the upper surface 121 of the projecting portion 120and in a direction (e.g., the W-axis direction in FIG. 98) perpendicularto the other pair of opposite sides of the upper surface 121. However,the angle made by the mirror 153 and the slant of the projecting portionL20 is small in directions the diagonal directions of the upper surfaces121 and 122) 45′ with respect the Z- and W-axes. As a result, thepull-in angle when the mirror 153 pivots in the directions 45° withrespect to the Z- and W-axes is smaller than that when the mirror 153pivots in the and W-axis directions. Samples with the structure shown inFIG. 96 were actually formed and measured. The pull-in angle is smallerby about 20% in pivotal movement in the directions 45° with respect tothe Z- and W-axes.

No problem arises if the pull-in angle is sufficiently large so that anoptical path can be formed at an arbitrary portion in a mirror array.However, an increase in the pull-in angle results in an increase in theangle of the slant of the projecting portion 120, i.e., an increase inthe difference of elevation between the terraces of the projectingportion 120. To increase the difference of elevation between theterraces of the projecting portion 120, the depth of anisotropic etchingmust be large. In addition, since the driving electrodes 103-1 to 103-4are formed on the surface of a structure with a large difference ofelevation, the manufacture is difficult. That is, the manufacture isvery difficult if the pull-in angle is ensured by the shape of theprojecting portion 120. Furthermore, to increase the pull-in angle, thedriving voltage must be high.

In fact, a design with a minimum necessary pull-in angle is required.That is, the design must tolerate that the pull-in angle, i.e., themaximum tilt angle of the mirror 153 has direction dependency dependingon the three-dimensional shape of the driving electrodes 103-1 to 103-4.Because of this direction dependency, for example, a region on theoutput-side mirror array scannable by the mirrors 153 of the input-sidemirror array is wide in the Z- and W-axis directions and narrow in thedirections 45° with respect to the Z- and W-axes.

In this embodiment, the parting lines of the driving electrodes 103-1 to103-4 are parallel to the Z- and W-axes, in addition to the arrangementof the 23rd embodiment. For example, as shown in FIG. 99, the drivingelectrodes 103-1 to 103-4 formed on and around the projecting portion120 have a first parting line (X-axis direction) parallel to the Z-axisand a second parting line (Y-axis direction) parallel to the W-axis.That is, the driving electrodes 103-1 to 103-4 are formed on the fourcorners of the projecting portion 120 and the lower substrate 101continuous from the four corners.

The above-described structure can maximize the range where the scannableregion (301 in FIG. 92) of the mirrors 153 decided by the directions ofthe parting lines of the driving electrodes 103-1 to 103-4 and tscannable region of the mirrors 153 decided by the three-dimensionalshape of the driving electrodes 103-1 to 103-4 overlap each other. Thatis, the ultimate scannable region of the mirrors 153 decided by the twokinds of scannable regions can be maximized. As a result, arrangementregions 2205 a and 2205 b of the mirror arrays 2203 a and 2203 b canhave a maximum area.

As described above, according to this embodiment, in four drivingelectrodes divisionally formed on a terrace-shaped projecting portionhaving an upper surface with an almost square shape when viewed from theupper side, a direction perpendicular to a pair of opposite sides of theupper surface of the projecting portion is set as a first parting line.A direction perpendicular to the other pair of opposite sides of theupper surface is set as a second parting line. When the electrodes areformed on the four corners of the projecting portion under theseconditions, the mirror arrangement regions of the first and secondmirror arrays can have a maximum area.

25th Embodiment

The 25th embodiment of the present invention will be described next.

Regarding a conventional optical switch as shown in FIG. 23, a reportsays that the flatness of a mirror 8230 influences connection loss andcrosstalk (Xiaoming Zhu and Joseph M. Kahn, Computing Insertion Loss inMEMS Optical Switches Caused By Non-Flat Mirrors, CLEO2001 CtuM43, May8, 2001, pp. 185-186). Hence, a mirror substrate 8201 of a conventionalmirror device 8200 as shown in FIG. 13 is made of single-crystal siliconwith excellent flatness. Since silicon has high transmittance in thecommunication wavelength band, a metal layer of, e.g., gold is generallyformed on the reflecting surface of the mirror 8230, as described above.The mirror 8230 is heated in forming the metal layer on it. If themirror 8230 stands at room temperature, the metal layer gets cool andshrinks. This produces internal stress between the metal layer andsilicon to make the mirror 8230 warp. Especially when a chromium layeris further provided to improve the adhesion between silicon and gold,the internal stress increases to make the warp large. There isconventionally required to control warp of the mirror 8230.

A low-loss optical switch can be formed not only by flattening themirror 8230 but also by optical design using a concave surface.Conventionally, however, the warp amount of the mirror 230 is hard tocontrol.

This embodiment has been made to solve the above-described problem andhas as its object to provide a mirror device manufacturing methodcapable of manufacturing a mirror with a desired warp amount.

This embodiment will be described next with reference to FIGS. 100A to100C. A mirror array to this embodiment includes a plurality of mirrordevices arranged two-dimensionally in a matrix and has a characteristicfeature in the structure of the mirror in the mirror device. The samenames and reference numerals as in the mirror device, mirror array, andoptical switch described in “Background Art” with reference to FIGS. 11,12, and 29 denote the same constituent elements in the mirror array ofthis embodiment, and a description thereof will be omitted as needed.

A mirror 230 in a mirror device included in mirror arrays 510 and 520 ofan optical switch 600 comprises a substrate layer 231, an upper surfacelayer 232 formed on a surface of the substrate layer 231 with aframe-shaped member 240, and a lower surface layer 233 formed on asurface of the substrate layer 231 opposite to the upper surface layer232.

The substrate layer 231 is made of, e.g., single-crystal silicon with analmost circular shape when viewed from the upper side and supported bymirror connectors 221 a and 221 b to be pivotal with respect to amovable frame 220.

The upper surface layer 232 includes a metal layer 232 a made of a metalsuch as gold, silver, or aluminum, and an intermediate layer 232 b madeof, e.g., chromium and provided between the metal layer 232 a and thesubstrate layer 231. The upper surface layer 232 has the same shape andsize as the substrate layer 231 and is formed to an arbitrary thickness.

The tower surface layer 233 includes a metal layer 233 a made of a metalsuch as gold, silver, or aluminum, and an intermediate layer 233 b madeof, e.g., chromium and provided between the metal layer 233 a and thesubstrate layer 231. The lower surface layer 233 has the same shape andsize as the substrate layer 231 and is formed to an arbitrary thickness.

The warp amount of the mirror 230 with the above-described structuredepends on the thicknesses of the upper surface layer 232 and lowersurface layer 233. This will be described below with reference to FIGS.101A to 102C. In FIGS. 101A and 101B, the ordinate represents the warpamount of the mirror 230, and the abscissa represents the distance inthe radial direction of the mirror 230 with a diameter of 600 μm. FIG.101A shows the warp amount on the lower surface side of the mirror 230when the upper surface layer 232 made of gold has a thickness of 0.23μm. FIG. 101B shows the warp amount on the lower surface side of themirror 230 when the upper surface layer 232 made of gold has a thicknessof 0.15 μm. FIGS. 101A and 101B show measurement results obtained by athree-dimensional surface structure analysis microscope “NewView200”available from Zygo.

Warp of the mirror 230 indicates that the flat mirror 230 curves, i.e.,the radius of curvature of the mirror 230 in an infinite statedecreases. The warp amount of the mirror 230 indicates the differencebetween the flat mirror 230 and the curved mirror 230 with warp in adirection perpendicular to the plane of the mirror 230 on the basis ofan end of the mirror 230 (the position “0” on the ordinate in FIGS. 101Aand 101B), as shown in FIG. 101.

As shown in FIGS. 101A and 101B, the warp amount of the mirror 230increases along with an increase in the thickness of the upper surfacelayer 232. For example, as shown in FIG. 101A, when the upper surfacelayer 232 is 0.23 μm thick, the warp amount of the mirror 230 exceeds0.2 μm at maximum. On the other hand, as shown in FIG. 101B, when theupper surface layer 232 is 0.15 μm thick, the warp amount of the mirror230 is smaller than 0.1 μm at maximum. Hence, it is possible to controlthe warp amount of the mirror 230 by controlling the thicknesses of theupper surface layer 232 and lower surface layer 233.

For example, when the upper surface layer 232 is formed thicker than thelower surface layer 233, as shown in FIG. 102A, the mirror 230 which hasa concave shape on the side of the upper surface layer 232, i.e., has aconcave surface warping to the side of the upper surface layer 232 canbe formed. Conversely, when the lower surface layer 233 is formedthicker than the upper surface layer 232, as shown in FIG. 102B, themirror 230 which has a concave shape on the side of the low surfacelayer 233, i.e., has a convex surface warping to the side of the lowersurface layer 233 can be formed. When the upper surface layer 232 andlower surface layer 233 have almost the same thickness, the flat mirror230 can be formed.

A method of manufacturing the mirror array according to this embodimentwill be described next with reference to FIGS. 103A to 103E. Thisembodiment has a characteristic feature in the structure of the mirrorin the mirror device included in the mirror array. The electrodesubstrate structure is the same as that in the mirror array described inthe above embodiments. Hence, a description of a method of manufacturingthe electrode substrate will be omitted as needed.

First, as shown in FIG. 103A, a side (to be referred to as an uppersurface hereinafter) of an SOI (Silicon On Insulator) substrate with aburied insulating layer 250 undergoes known photolithography and etchingsuch as DEEP RIE to form, in the single-crystal silicon layer, trenchesconforming to the shapes of a frame portion 210, movable frameconnectors 211 a and 211 b, movable frame 220, mirror connectors 221 aand 221 b, and mirror 230.

As shown in FIG. 103B, a resist pattern with openings in predeterminedregions corresponding to the trenches is formed on the lower surface ofthe SOI substrate. The silicon is selectively etched from the lowersurface of the SOI substrate by using an et chant such as a potassiumhydroxide solution. In this etching, an opening and the frame-shapedmember 240 are formed on the lower surface of the SOI substrate by usingthe buried insulating layer 250 as an etching stopper layer.

As shown in FIG. 103C, a region of the buried insulating layer 250exposed to the opening is removed by using hydrofluoric acid.

As shown in FIG. 103D, a mask with an opening in a predetermined regioncorresponding to the mirror 230 is formed on the upper surface of theSOI substrate. The intermediate layer 232 b and metal layer 232 a aresequentially formed by known vapor deposition or sputtering to form theupper surface layer 232. At this time, the metal layer 232 a is formedto an arbitrary thickness such that the mirror 230 has a desired warpamount, i.e., a desired radius of curvature.

As shown in FIG. 103E, a mask with an opening in a predetermined regioncorresponding to the mirror 230 is formed on the lower surface of theSOI substrate. The intermediate layer 233 b and metal layer 233 a aresequentially formed by known vapor deposition or sputtering to form thelower surface layer 233. At this time, the metal layer 233 a is formedto an arbitrary thickness such that the mirror 230 has a desired warpamount, i.e., a desired radius of curvature.

The radius of curvature of the mirror 230 is measured. If the measuredradius of curvature is different from the desired radius of curvature,at least one of the metal layers 232 a and 233 a is further formed onthe mirror 230 on the basis of the difference in accordance with thesame procedures as described with reference to FIGS. 103D and 103E. Withthis process, the mirror 230 having the desired shape is formed.

As described above, according to this embodiment, the warp amount of themirror 230 is controlled by controlling the thickness of at least one ofthe upper surface layer 232 and lower surface layer 233. This allows tomanufacture the mirror 230 having a desired radius of curvature.

In this embodiment, after forming the upper surface layer 232 and lowersurface layer 233 or after joining a mirror substrate 200 with the uppersurface layer 232 and lower surface layer 233 to an electrode substrate300, annealing may be done by heating the mirror substrate 230 and thenslowly cooling it. The annealing temperature at this time is equal to orhigher than a higher one of the temperatures in forming the uppersurface layer 232 and lower surface layer 233. This allows to controlthe shape of the mirror 230 without any influence of the variations inthe temperatures in forming the upper surface layer 232 and lowersurface layer 233.

26th Embodiment

The 26th embodiment of the present invention will be described next.This embodiment controls the warp amount of a mirror 230 by changing thetemperature in forming an upper surface layer 232 and lower surfacelayer 233. The same names and reference numerals as in the 25thembodiment denote the same constituent elements the 26th embodiment, anda description thereof will be omitted as needed.

A substrate layer 231 is made of, e.g., single-crystal silicon with analmost circular shape when viewed from the upper side and supported bymirror connectors 221 a and 221 b to be pivotal with respect to amovable frame 220.

The upper surface layer 232 includes a metal layer 232 a made of a metalsuch as gold, silver, or aluminum, and an intermediate layer 232 b madeof, e.g., chromium and provided between the metal layer 232 a and thesubstrate layer 231. The upper surface layer 232 has the same shape andsize as the substrate layer 231 and is formed under an arbitrarytemperature.

The lower surface layer 233 includes a metal layer 233 a made of a metalsuch as gold, silver, or aluminum, and an intermediate layer 233 b madeof, e.g., chromium and provided between the metal layer 233 a and thesubstrate layer 231. The lower surface layer 233 has the same shape andsize as the substrate layer 231 and is formed under an arbitrarytemperature.

The warp amount of the mirror 230 also depends on the temperature informing the upper surface layer 232 and lower surface layer 233. Thelarger the difference between the room temperature and the temperaturein forming the upper surface layer 232 and lower surface layer 233becomes, the larger the warp amount of the mirror 230 becomes. Forexample, when the temperature in forming the upper surface layer 232 ishigher than that in forming the lower surface layer 233, the mirror 230having a concave surface warping to the side of the upper surface layer232 can be formed, as shown in FIG. 104A. Conversely, when thetemperature in forming the lower surface layer 233 is higher than thatin forming the upper surface layer 232, the mirror 230 having a convexsurface warping to the side of the lower surface layer 233 can beformed, as shown in FIG. 104B. When the upper surface layer 232 andlower surface layer 233 are formed at almost the same temperature, theflat mirror 230 can be formed, as shown in FIG. 104C.

A method of manufacturing a mirror array according to this embodimentwill be described next with reference to FIGS. 103A to 103E.

First, as shown in FIG. 103A, a side (to be referred to as an uppersurface hereinafter) of an SOI (Silicon On Insulator) substrate with aburied insulating layer 250 undergoes known photolithography and etchingsuch as DEEP RIE to form, in the single-crystal silicon layer, trenchesconforming to the shapes of a frame portion 210, movable frameconnectors 211 a and 211 b, movable frame 220, mirror connectors 221 aand 221 b, and mirror 230.

As shown in FIG. 103B, a resist pattern with openings in predeterminedregions corresponding to the trenches is formed on the lower surface ofthe SOI substrate. The silicon is selectively etched from the lowersurface of the SOI substrate by using an etchant such as a potassiumhydroxide solution. In this etching, an opening and the frame-shapedmember 240 are formed on the lower surface of the SOI substrate by usingthe buried insulating layer 250 as an etching stopper layer.

As shown in FIG. 103C, a region of the buried insulating layer 250exposed to the opening is removed by using hydrofluoric acid.

Next, a metal layer is formed on a surface of the mirror 230, whichshould change to a concave surface, i.e., warp. For example, to make theside of the upper surface layer 232 warp, a mask with an opening in apredetermined region corresponding to the mirror 230 is formed on theupper surface of the SOI substrate, as shown in FIG. 103D. Theintermediate layer 232 b and metal layer 232 a are sequentially formedby known vapor deposition or sputtering to form the upper surface layer232. At this time, the metal layer 232 a is formed at a first arbitrarytemperature such that the mirror 230 has a desired warp amount, i.e., adesired radius of curvature. For example, the susceptor to fix the SOIsubstrate in the apparatus for forming the metal layer 232 a has aheater. The heater heats the SOI substrate to an arbitrary temperature,and the metal layer 232 a is formed under this condition. The firstarbitrary temperature is set to be higher than a second arbitrarytemperature (to be described later).

A metal layer is formed on a surface of the mirror 230, which shouldchange to a convex surface. For example, to form a convex surface on theside of the lower surface layer 233, a mask with an opening in apredetermined region corresponding to the mirror 230 is formed on thelower surface of the SOI substrate, as shown in FIG. 103E. Theintermediate layer 233 b and metal layer 233 a are sequentially formedby known vapor deposition or sputtering to form the lower surface layer233. At this time, the metal layer 233 a is formed at a second arbitrarytemperature such that the mirror 230 has a desired warp amount, i.e., adesired radius of curvature. For example, the susceptor to fix the SOIsubstrate in the apparatus for forming the metal layer 233 a has aheater. The heater heats the SOI substrate to an arbitrary temperature,and the metal layer 233 a is formed under this condition.

With this process, the mirror 230 having the desired shape is formed. Asdescribed above, according to this embodiment, the warp amount of themirror 230 is controlled by controlling the temperature in forming atleast one of the upper surface layer 232 and lower surface layer 233.This allows to manufacture the mirror 230 having a desired radius ofcurvature.

To form the flat mirror 230, the second arbitrary temperature is set tobe higher than the first arbitrary temperature. Since each of the uppersurface layer 232 and lower surface layer 233 is heated to the secondarbitrary temperature and then cooled down to the room temperature, theflat mirror 230 becomes flat.

In this embodiment, the thickness of at least one of the upper surfacelayer 232 and lower surface layer 233 may be controlled, as in the 25thembodiment. This enables to more accurately control the shape of themirror 230.

27th Embodiment

The 27th embodiment of the present invention will be described next.This embodiment controls the warp amount of a mirror 230 by setting thematerials of an upper surface layer 232 and lower surface layer 233 onthe basis of the coefficient of thermal expansion.

The same names and reference numerals as in the 25th and 26thembodiments denote the same constituent elements the 27th embodiment,and a description thereof will be omitted as needed.

A substrate layer 231 is made of, e.g., single-crystal silicon with analmost circular shape when viewed from the upper side and supported bymirror connectors 221 a and 221 b to be pivotal with respect to amovable frame 220.

The upper surface layer 232 includes a metal layer 232 a made of one ofmetals such as gold, silver, and aluminum in accordance with the shapeof the mirror 230, and an intermediate layer 232 b made of, e.g.,chromium and provided between the metal layer 232 a and the substratelayer 231. The upper surface layer 232 has the same shape and size asthe substrate layer 231.

The lower surface layer 233 includes a metal layer 233 a made of one ofmetals such as gold, silver, and aluminum in accordance with the shapeof the mirror 230, and an intermediate layer 233 b made of, e.g.,chromium and provided between the metal layer 233 a and the substratelayer 231. The lower surface layer 233 has the same shape and size asthe substrate layer 231.

The warp amount of the mirror 230 also depends on the coefficients ofthermal expansion of the materials of the upper surface layer 232 andlower surface layer 233. As described above, when the upper surfacelayer 232 or lower surface layer 233 heated in the formation processgets cool to the room temperature and shrinks, the mirror 230 warps. Ifthe coefficient of thermal expansion of the material changes between theupper surface layer 232 and the lower surface layer 233, the shrinkageamount upon cooling also changes. Hence, the warp amount of the mirror230 also changes. As described above, this embodiment controls the warpamount of the mirror 230 by setting the materials of the upper surfacelayer 232 and lower surface layer 233 on the basis of the coefficient ofthermal expansion.

For example, when the coefficient of thermal expansion of the materialof the upper surface layer 232 is larger than that of the lower surfacelayer 233, the mirror 230 having a concave surface warping to the sideof the upper surface layer 232 can be formed, as shown in FIG. 104A.Conversely, when the coefficient of thermal expansion of the material ofthe lower surface layer 233 is larger than that of the upper surfacelayer 232, the mirror 230 having a convex surface warping to the side ofthe lower surface layer 233 can be formed, as shown in FIG. 104B. Whenthe upper surface layer 232 and lower surface layer 233 are made of thesame material, the flat mirror 230 can be formed, as shown in FIG. 104C.

A method of manufacturing a mirror array according to this embodimentwill be described next with reference to FIGS. 103A to 103E.

First, as shown in FIG. 103A, a side (to be referred to as an uppersurface hereinafter) of an SOI (Silicon On Insulator) substrate with aburied insulating layer 250 undergoes known photolithography and etchingsuch as DEEP RIE to form, in the single-crystal silicon layer, trenchesconforming to the shapes of a frame portion 210, movable frameconnectors 211 a and 211 b, movable frame 220, mirror connectors 221 aand 221 b, and mirror 230.

As shown in FIG. 103B, a resist pattern with openings in predeterminedregions corresponding to the trenches is formed on the lower surface ofthe SOI substrate. The silicon is selectively etched from the lowersurface of the SOI substrate by using an etchant such as a potassiumhydroxide solution. In this etching, an opening and the frame-shapedmember 240 are formed on the lower surface of the SOI substrate by usingthe buried insulating layer 250 as an etching stopper layer.

As shown in FIG. 103C, a region of the buried insulating layer 250exposed to the opening is removed by using hydrofluoric acid.

As shown in FIG. 103D, a mask with an opening in a predetermined regioncorresponding to the mirror 230 is formed on the upper surface of theSOI substrate. The intermediate layer 232 b and metal layer 232 a aresequentially formed by known vapor deposition or sputtering to form theupper surface layer 232. At this time, the metal layer 232 a is made ofa material having an arbitrary coefficient of thermal expansion suchthat the mirror 230 has a desired warp amount, i.e., a desired radius ofcurvature.

As shown in FIG. 103E, a mask with an opening in a predetermined regioncorresponding to the mirror 230 is formed on the lower surface of theSOI substrate. The intermediate layer 233 b and metal layer 233 a aresequentially formed by known vapor deposition or sputtering to form thelower surface layer 233. At this time, the metal layer 233 a is made ofa material having an arbitrary coefficient of thermal expansion suchthat the mirror 230 has a desired warp amount, i.e., a desired radius ofcurvature. With this process, the mirror 230 having the desired shape isformed.

As described above, according to this embodiment, the warp amount of themirror 230 is controlled by setting the materials of the upper surfacelayer 232 and lower surface layer 233 on the basis of the coefficient ofthermal expansion. This allows to manufacture the mirror 230 having adesired radius of curvature.

In this embodiment, after forming the upper surface layer 232 and lowersurface layer 233 or after joining a mirror substrate 200 with the uppersurface layer 232 and lower surface layer 233 to an electrode substrate300, annealing may be done by heating the mirror substrate 230 and thenslowly cooling it. The annealing temperature at this time is equal to orhigher than a higher one of the temperatures in forming the uppersurface layer 232 and lower surface layer 233. This allows to controlthe shape of the mirror 230 without any influence of the variations inthe temperatures in forming the upper surface layer 232 and lowersurface layer 233.

In this embodiment, the thickness of at least one of the upper surfacelayer 232 and lower surface layer 233 may also be controlled, as in the25th embodiment. This enables to more accurately control the shape ofthe mirror 230.

In this embodiment, the temperature in forming the upper surface layer232 and lower surface layer 233 may also be controlled, as in the 26thembodiment. This also enables to more accurately control the shape ofthe mirror 230.

In the 25th to 27th embodiments, the warp amount of the mirror 230 iscontrolled on the basis of one of the thickness of the metal layers 232a and 233 a, the temperature in the formation process, and thecoefficient of thermal expansion. However, the warp amount of the mirror230 may be controlled on the basis of all of these factors. This enablesto more accurately control the shape of the mirror 230.

In the 25th to 27th embodiments, the intermediate layers 232 b and 233 bare inserted between the substrate layer 231 and the metal layers 232 aand 233 a. The intermediate layers 232 b and 233 b may be omitted.

In the 25th to 27th embodiments, the lower surface layer 233 is formedafter formation of the upper surface layer 232. The order of formationmay be reversed.

The mirror arrays and mirror devices according to the 25th to 27thembodiments are usable not only in an optical switch but also in ameasurement device, display, and scanner.

As described above, according to the 25th to 27th embodiments, it ispossible to control warp of the mirror by providing a metal layer notonly on a surface but also on the other surface of the mirror.

28th Embodiment

The 28th embodiment of the present invention will be described next.

A method of manufacturing a conventional mirror device shown in FIGS. 11and 12 will briefly be described. A mirror substrate 8201 can be formedfrom an SOI (Silicon On Insulator) substrate. The SOI substrate has athin silicon layer (SOI layer) on a buried insulating layer on a thicksilicon base. The above-described structures such as a base 8210,movable frame 8220, and mirror 8230 can be formed by fabricating the SOIlayer. When the thick base of the SOI substrate is removed to leave aframe shape, a frame portion 240 can be formed. An insulating layer 8241shown in FIGS. 11 and 12 corresponds to the buried insulating layer ofthe SOI substrate.

An electrode substrate 8301 can be formed by etching a single-crystalsilicon substrate with a crystal orientation (100) on the major surfaceby using an alkaline solution such as a potassium hydroxide solution.The etching rate of single-crystal silicon by alkali is much lower inthe (111) plane than in the (100) and (110) planes. A projecting portion8320 with a truncated pyramidal shape and convex portions (ribstructures) 8360 a and 8360 b can be formed by using this phenomenon.

The mirror substrate 8201 and electrode substrate 8301 are formed in theabove-described way and bonded to each other, thereby forming a mirrordevice which makes the mirror 8230 move (pivot) by applying an electricfield to electrodes 8340 a to 8340 d, as shown in FIG. 12. To improvethe reflectance of the mirror 8230, a metal layer of, e.g., gold isformed on the surface (the surface shown in FIGS. 11 and 12) of themirror 8230.

After formation of, e.g., the mirror portion connected by mirrorconnectors, the mirror substrate formed in the above-described manner ishandled while keeping the mirror in a pivotable state. For example, inthe step (substrate rotation by spin coating, substrate conveyancebetween manufacturing apparatuses, and substrate cleaning) of formingstructures included in the mirror by patterning or selective etching,the wafer dicing step, the step of forming a metal layer on the mirrorsurface, the step of bonding the mirror substrate to a substrate withelectrode interconnections for driving the mirror, the step ofdie-bonding the structure to a package, the wire bonding step, and thepotting step, the mirror substrate is handled while keeping the movableportions such as the mirror and movable frame connected by the fragileconnectors.

In the above-described mirror device, an electric field generated by avoltage applied to the electrodes formed on the electrode substrategives an attracting force to the mirror and rotates it by severaldegrees. To control the pivotal movement (position) of the mirror athigh positional accuracy, the connectors are designed to deform by aslight force. For example, the connectors include torsion springs with awidth of 2 μm and a thickness of 10 μm. The connectors readily breakupon receiving a large force. The mirror itself that is formed thin alsoreadily breaks or chips upon receiving a large force.

In the above-described processes, a water current, a centrifugal forcein drying a wafer, vibration or impact is generated and applied to theconnectors and mirror. Hence, the connectors easily break, or the mirroreasily chips. As a result, the manufacturing yield of mirror substratesdecreases. Especially, a mirror ray including a number of mirror devicesarrayed in a matrix becomes defective if one mirror is defective,resulting in a further decrease in yield.

This embodiment has been made to solve the above-described problem andhas as its object to form mirror substrates at a high yield ofnon-defective units.

This embodiment will be described next with reference to FIGS. 105A to105K. First, as shown in 105A, an SOI substrate having, on a siliconbase 2301 with a plane orientation (100), a buried insulating layer 2302made of silicon oxide and having a thickness of about 1 μm, and a 10-μmthick single-crystal silicon layer (SOI layer) 2303 is prepared. Anoxide layer 2304 is formed on the surface of the SOI layer 2303, and anoxide layer 2305 is formed on the lower surface of the silicon base 2301by, e.g., thermal oxidation.

As shown in FIG. 105B, a resist mask layer 2306 with a photoresistpattern formed by known photolithography is formed on the oxide layer2304. The oxide layer 2304 is etched by using the resist mask layer 2306as a mask. At this time, directional etching such as reactive ionetching is performed to expose the surface of the SOI layer 2303 at theetched portions. With this process, an inorganic mask layer (movableportion formation mask pattern) 2304 a with a mask pattern of siliconoxide is formed, as shown in FIG. 105C. At this time, a pattern to formscribe lines serving as a guide in dicing is provided in a region (notshown) of the resist mask layer 2306.

The resist mask layer 2306 is removed by ashing using ozone or oxygenplasma. As shown in FIG. 105D, the SOI layer 2303 is etched by dryetching using the inorganic mask 2304 a as a mask. With this etching,the base 2301, movable frame 2303, mirror (mirror structure) 2305,connectors (not shown), and mirror connectors (not shown) are formed.That is, the basic structure of the mirror substrate is complete. Thescribe line pattern formed in the region (not shown) of the resist masklayer 2306 is also transferred to the inorganic mask layer 2304 a andthen to the SOI layer 2303. The mirror structure may connect to the basewithout the movable frame.

A resin is applied onto the inorganic mask 2304 a to form a resin film2307 that fills the spaces between the patterns of the inorganic mask2304 a and the spaces between the structures formed in the SOI layer2303, as shown in FIG. 105E. The resin film 2307 is etched back to forma protective layer 2307 a that exposes the surface of the inorganic masklayer 2304 a and fills the spaces between the patterns of the inorganicmask 2304 a and the spaces between the structures formed in the SOIlayer 2303, as shown in FIG. 105F.

The inorganic mask layer 2304 a is removed by, e.g., known chemicalmechanical polishing (CMP) to form the protective layer 2307 a thatfills the spaces between the structures formed in the SOI layer 2303with a flat surface, as shown in FIG. 105G. As will be described below,in etching the silicon base 2301, the surface of the SOI layer 2303 isfixed in each processing apparatus. For this reason, the surface of theSOI layer 2303 is preferably flat after the inorganic mask layer 2304 ais removed.

Next, the oxide layer 2305 and silicon base 2301 are etched by using amask pattern (frame formation mask pattern) formed by knownphotolithography to form a frame portion 2301 a, as shown in FIG. 105H.The mask pattern is removed. Then, the oxide layer 2305 and the buriedinsulating layer 2302 exposed inside the frame portion 2301 a areremoved by, e.g., wet etching using an alkaline solution or dry etching,as shown in FIG. 105I.

As shown in FIG. 105J, a metal film 2308 of, e.g., Au is formed on thesurface of the mirror 2335 with the frame portion 2301 d by, e.g., vapordeposition. The metal film 2308 is selectively formed at the portion ofthe mirror 2335 by using, e.g., a stencil mask. A step of bonding amirror substrate 2300 to the electrode substrate to form a mirrordevice, a step of packaging the mirror device and fixing it by diebonding, and a step of wire-bonding the terminals of the package to theterminals of the electrode substrate are executed. Then, the protectivelayer 2307 a is removed by, e.g., ashing using oxygen plasma to form aspace between the base 2301, movable frame 2302, and mirror 2335 to makethe movable frame 2302 and mirror 2335 pivotable, as partially shown inFIG. 105K. The protective layer 2307 a may be removed after formation ofthe mirror device or between the above-described packaging steps.

FIG. 106 is a perspective view schematically showing the schematicstructure of the mirror substrate 2300 formed by the steps described inFIGS. 105A to 105I. As shown in FIG. 106, the mirror substrate 2300comprises the base 2301, movable frame 2303, and mirror 2335 formed inthe SOI layer 2303. The movable frame 2303 is arranged in the opening ofthe base 2301 and connects to the base 2301 through a pair of connectors2332 a and 2332 b. The mirror 2335 is arranged in the opening of themovable frame 2303 and connects to the movable frame 2303 through a pairof mirror connectors 2334 a and 2334 b. The frame portion 2301 a formedaround the base 2301 surrounds the movable frame 2303 and mirror 2335.The frame portion 2301 a is fixed to the base 2301 through the buriedinsulating layer 2302.

The connectors 2332 a and 2332 b provided in the notches of the movableframe 2303 connect the base 2301 to the movable frame 2303. The movableframe 2303 connected to the base 2301 can pivot about the pivot axis(movable frame pivot axis) passing through the connectors 2332 a and2332 b. The mirror connectors 2334 a and 2334 b provided in the notchesof the movable frame 2303 connect the movable frame 2303 to the mirror2335. The mirror 2335 connected to the movable frame 2303 can pivotabout the mirror pivot axis passing through the mirror connectors 2334 aand 2334 b.

The above-described structure is the same as that of the mirrorsubstrate 8201 shown in FIG. 11. The mirror substrate 2300 shown in FIG.106 comprises the protective layer 2307 a that fills the gaps betweenthe base 2301, connectors 2332 a and 2332 b, movable frame 2303, mirrorconnectors 2334 a and 2334 b, and mirror 2335. Hence, the mirrorsubstrate 2300 shown in FIG. 106 suppresses motions such as pivotalmovement in the above-described structure so that it is protected fromdamage or breakage due to external mechanical vibration.

For example, it is possible to suppress damage to the connectors evenwhen an external mechanical vibration is added in loading and fixing themirror substrate 2300 in a vapor deposition apparatus to form the metalfilm 2308 described with reference to FIG. 105J. Similarly, it ispossible to suppress damage to the connectors in the step of bonding themirror substrate 2300 to the electrode substrate to form the mirrordevice, the step of packaging the mirror device and fixing it by diebonding, and the step of wire-bonding the terminals of the package tothe terminals of the electrode substrate.

As described above, according to this embodiment, when the base,connectors, and mirror structure are formed on the buried insulatinglayer, a protective layer filling the spaces between them is formed.Even when the buried insulating layer in the mirror formation region isremoved to expose the both surfaces of the silicon layer and make themirror structure movable, the mirror structure is prevented from moving.As a result, according to the present invention, the mirror structureand connectors are protected from damage. This allows to form mirrorsubstrates at a high yield of non-defective units.

The eighth to 28th embodiments may have the antistatic structuredescribed in the first to seventh embodiments.

INDUSTRIAL APPLICABILITY

The present invention is applicable, to a electrostatically drivendevice having a mirror with a changeable tilt angle, a mirror arrayhaving a plurality of mirror devices arranged two-dimensionally, anoptical switch having the mirror array, a method of manufacturing amirror substrate included in the mirror device, and a method ofmanufacturing the mirror device.

The invention claimed is:
 1. A mirror device comprising: a mirror whichis supported to be pivotable with respect to a mirror substrate; adriving electrode which is formed on an electrode substrate facing saidmirror substrate; an antistatic structure which is arranged in a spacebetween said mirror and said electrode substrate; an interconnectionwhich is formed on said electrode substrate to supply a driving voltageto said driving electrode; and a conductive member which isequipotential to said mirror and arranged at a position closer to saidinterconnection than said mirror, wherein said driving electrode andsaid interconnection are formed on an insulating layer on said electrodesubstrate having conductivity, and said electrode substrate comprisessaid a conductive member equipotential to said mirror.